Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012
Liabilities:
Derivative liabilities	$ 12,400
Carrying Amount
Assets:
Cash and cash equivalents	648,479
Carrying Amount | Interest rate swap agreements
Liabilities:
Derivative liabilities	11,532
Carrying Amount | Natural gas swap agreements
Liabilities:
Derivative liabilities	838
Carrying Amount | Bank debt
Liabilities:
Long term debt fair value	1,166,341
Carrying Amount | 7 1/4% Senior Notes
Liabilities:
Long term debt fair value	245,454
Carrying Amount | 5% Senior Notes
Liabilities:
Long term debt fair value	500,000
Fair Value
Assets:
Cash and cash equivalents	648,479
Fair Value | Interest rate swap agreements
Liabilities:
Derivative liabilities	11,532
Fair Value | Natural gas swap agreements
Liabilities:
Derivative liabilities	838
Fair Value | Bank debt
Liabilities:
Long term debt fair value	1,166,341
Fair Value | 7 1/4% Senior Notes
Liabilities:
Long term debt fair value	280,938
Fair Value | 5% Senior Notes
Liabilities:
Long term debt fair value	$ 498,440