Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Sep. 09, 2013	Mar. 23, 2012
Carrying Amount
Assets:
Cash and cash equivalents	$ 99,945	$ 222,591
Carrying Amount | Interest rate swap agreements
Liabilities:
Derivative liabilities	771	1,196
Carrying Amount | Natural Gas [Member]
Liabilities:
Derivative liabilities	813	746
Fair Value
Assets:
Cash and cash equivalents	99,945	222,591
Fair Value | Interest rate swap agreements
Liabilities:
Derivative liabilities	771	1,196
Fair Value | Natural Gas [Member]
Liabilities:
Derivative liabilities	813	746
Senior Notes [Member] | 5½% Senior Notes [Member]
Liabilities:
Stated interest rate			5.50%
Senior Notes [Member] | 5% Senior Notes due 2020 [Member]
Liabilities:
Stated interest rate				5.00%
Senior Notes [Member] | Carrying Amount | 5½% Senior Notes [Member]
Liabilities:
Long term debt fair value	300,000	300,000
Senior Notes [Member] | Carrying Amount | 5% Senior Notes due 2020 [Member]
Liabilities:
Long term debt fair value	500,000	500,000
Senior Notes [Member] | Fair Value | 5½% Senior Notes [Member]
Liabilities:
Long term debt fair value	309,189	307,500
Senior Notes [Member] | Fair Value | 5% Senior Notes due 2020 [Member]
Liabilities:
Long term debt fair value	510,055	500,715
Bank Borrowings [Member] | Carrying Amount
Liabilities:
Long term debt fair value	725,818	798,963
Bank Borrowings [Member] | Fair Value
Liabilities:
Long term debt fair value	725,818	798,963
Other Liabilities [Member] | Interest rate swap agreements
Liabilities:
Derivative liabilities	$ 100	$ 100