Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Carrying Amount
Assets:
Cash and cash equivalents	$ 397,101	$ 175,226
Carrying Amount | Interest rate swap agreements
Liabilities:
Derivative liabilities	11,239	11,655
Carrying Amount | Natural gas swap agreements
Liabilities:
Derivative liabilities	797	28
Carrying Amount | Bank debt
Liabilities:
Long term debt fair value	1,131,022	660,262
Carrying Amount | 7 1/4% Senior Notes
Liabilities:
Long term debt fair value	245,237	244,412
Fair Value
Assets:
Cash and cash equivalents	397,101	175,226
Fair Value | Interest rate swap agreements
Liabilities:
Derivative liabilities	11,239	11,655
Fair Value | Natural gas swap agreements
Liabilities:
Derivative liabilities	797	28
Fair Value | Bank debt
Liabilities:
Long term debt fair value	1,131,022	660,262
Fair Value | 7 1/4% Senior Notes
Liabilities:
Long term debt fair value	$ 267,500	$ 267,500