Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Carrying Amount
Assets:
Cash and cash equivalents	$ 465,608	$ 397,101
Carrying Amount | Interest rate swap agreements
Liabilities:
Derivative liabilities	13,307	11,239
Carrying Amount | Natural gas swap agreements
Liabilities:
Derivative liabilities	279	797
Carrying Amount | Bank debt
Liabilities:
Long term debt fair value	1,171,316	1,131,022
Carrying Amount | 5% Senior Notes
Liabilities:
Long term debt fair value	500,000
Fair Value
Assets:
Cash and cash equivalents	465,608	397,101
Fair Value | Interest rate swap agreements
Liabilities:
Derivative liabilities	13,307	11,239
Fair Value | Natural gas swap agreements
Liabilities:
Derivative liabilities	279	797
Fair Value | Bank debt
Liabilities:
Long term debt fair value	1,171,316	1,131,022
Fair Value | 5% Senior Notes
Liabilities:
Long term debt fair value	$ 522,500