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Long-Term Debt - Additional Information (Detail) (USD $)
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12 Months Ended | ||
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Mar. 31, 2013
Y
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Mar. 31, 2012
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Mar. 31, 2011
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| Long Term Debt And Equity Financings [Line Items] | |||
| New facilities, line of credit | $ 405,000,000 | ||
| Amounts drawn under the combined facilities | 0 | ||
| Weighted average interest rate on total debt | |||
| Weighted average term of debt facilities | 3.1 | ||
| Proceeds from long-term borrowings | 330,000,000 | 160,000,000 | 460,000,000 |
| Repayments of long-term borrowings | 330,000,000 | 219,000,000 | 555,000,000 |
| Debt instrument, description of variable rate | LIBOR | ||
| Debt covenant description in credit facility agreement | At 31 March 2013, the Company was in compliance with all restrictive debt covenants contained in its credit facility agreements. Under the most restrictive of these covenants, the Company (i) must not exceed a maximum of net debt to earnings before interest, tax, depreciation and amortisation, excluding all income, expense and other profit and loss statement impacts of AICF, Amaba, Amaca, ABN 60 and Marlew Mining Pty Limited and excluding assets, liabilities and other balance sheet items of the AICF, Amaba, Amaca, ABN 60 and Marlew Mining Pty Limited, (ii) must meet or exceed a minimum ratio of earnings before interest and taxes to net interest charges, excluding all income, expense and other profit and loss statement impacts of AICF, Amaba, Amaca, ABN 60 and Marlew Mining Pty Limited, and (iii) must ensure that no more than 35% of Free Cash Flow (as defined in the AFFA), in any given financial year (“Annual Cash Flow Cap”) is contributed to AICF on the payment dates under the AFFA in the next following financial year. The Annual Cash Flow Cap does not apply to payments of interest, if any, to AICF and is consistent with contractual obligations of the Performing Subsidiary and the Company under the AFFA. | ||
| Maximum percentage of Free Cash Flow contributed to AICF | 35.00% | ||
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Term facilities [Member]
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| Long Term Debt And Equity Financings [Line Items] | |||
| Proceeds from long-term borrowings | 330,000,000 | ||
| Repayments of long-term borrowings | 330,000,000 | ||
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Line Of Credit Facility Matured February And March Two Thousand And Thirteen Member [Member]
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| Long Term Debt And Equity Financings [Line Items] | |||
| Line of credit facility Cancelled Borrowing Capacity | 230,000,000 | ||
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Line Of Credit Facility New Facilities [Member]
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| Long Term Debt And Equity Financings [Line Items] | |||
| New facilities, line of credit | 355,000,000 | ||
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Facilities mature in March 2016 [Member]
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| Long Term Debt And Equity Financings [Line Items] | |||
| New facilities, line of credit | 50,000,000 | ||
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Facilities mature in April 2016 [Member]
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| Long Term Debt And Equity Financings [Line Items] | |||
| New facilities, line of credit | 190,000,000 | ||
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Facilities mature in March 2017 [Member]
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| Long Term Debt And Equity Financings [Line Items] | |||
| New facilities, line of credit | 40,000,000 | ||
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Facilities mature in April 2017 [Member]
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| Long Term Debt And Equity Financings [Line Items] | |||
| New facilities, line of credit | $ 75,000,000 | ||