Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013 Carrying Amount	Dec. 31, 2012 Carrying Amount	Dec. 31, 2013 Carrying Amount Interest rate swap agreements	Dec. 31, 2012 Carrying Amount Interest rate swap agreements	Dec. 31, 2013 Carrying Amount Natural gas swap agreements	Dec. 31, 2012 Carrying Amount Natural gas swap agreements	Dec. 31, 2013 Fair Value	Dec. 31, 2012 Fair Value	Dec. 31, 2013 Fair Value Interest rate swap agreements	Dec. 31, 2012 Fair Value Interest rate swap agreements	Dec. 31, 2013 Fair Value Natural gas swap agreements	Dec. 31, 2012 Fair Value Natural gas swap agreements	Sep. 09, 2013 Senior Notes [Member] 5½% Senior Notes [Member]	Mar. 23, 2012 Senior Notes [Member] 5% Senior Notes due 2020 [Member]	Dec. 31, 2013 Senior Notes [Member] Carrying Amount 5½% Senior Notes [Member]	Dec. 31, 2012 Senior Notes [Member] Carrying Amount 5½% Senior Notes [Member]	Dec. 31, 2013 Senior Notes [Member] Carrying Amount 5% Senior Notes due 2020 [Member]	Dec. 31, 2012 Senior Notes [Member] Carrying Amount 5% Senior Notes due 2020 [Member]	Dec. 31, 2013 Senior Notes [Member] Fair Value 5½% Senior Notes [Member]	Dec. 31, 2012 Senior Notes [Member] Fair Value 5½% Senior Notes [Member]	Dec. 31, 2013 Senior Notes [Member] Fair Value 5% Senior Notes due 2020 [Member]	Dec. 31, 2012 Senior Notes [Member] Fair Value 5% Senior Notes due 2020 [Member]	Dec. 31, 2013 Bank Borrowings [Member] Carrying Amount	Dec. 31, 2012 Bank Borrowings [Member] Carrying Amount	Dec. 31, 2013 Bank Borrowings [Member] Fair Value	Dec. 31, 2012 Bank Borrowings [Member] Fair Value
Assets:
Cash and cash equivalents	$ 160,463	$ 465,608					$ 160,463	$ 465,608
Liabilities:
Long term debt fair value															300,000	0	500,000	500,000	298,125	0	495,625	522,500	903,836	1,171,316	903,836	1,171,316
Stated interest rate													5.50%	5.00%
Derivative liabilities			$ 6,895	$ 13,307	$ 22	$ 279			$ 6,895	$ 13,307	$ 22	$ 279