Summary of Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Detail) $ in Thousands, MMBTU in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MMBTU	Dec. 31, 2015 USD ($) MMBTU	Sep. 09, 2013	Mar. 23, 2012
Interest rate swap agreements
Liabilities:
Derivative liabilities	$ 50,000
Derivative Maturity Month And Year	2017
Interest Rate Swap [Member]
Liabilities:
Derivative liabilities	$ 100,000
Natural Gas Commodity [Member]
Liabilities:
Derivative, Nonmonetary Notional Amount | MMBTU	1.4	1.4
Carrying Amount
Assets:
Cash and cash equivalents	$ 24,690	$ 99,945
Carrying Amount | Natural Gas [Member]
Assets:
Cash Flow Hedge Derivative Instrument Assets at Fair Value	1,094
Liabilities:
Cash Flow Hedge Derivative Instrument Liabilities at Fair Value	0	813
Carrying Amount | Interest Rate Swap [Member]
Liabilities:
Cash Flow Hedge Derivative Instrument Liabilities at Fair Value	244	771
Fair Value
Assets:
Cash and cash equivalents	24,690	99,945
Fair Value | Natural Gas [Member]
Assets:
Cash Flow Hedge Derivative Instrument Assets at Fair Value	1,094
Liabilities:
Cash Flow Hedge Derivative Instrument Liabilities at Fair Value	0	813
Fair Value | Interest Rate Swap [Member]
Liabilities:
Cash Flow Hedge Derivative Instrument Liabilities at Fair Value	244	771
Senior Notes [Member] | 5½% Senior Notes [Member]
Liabilities:
Stated interest rate			5.50%
Senior Notes [Member] | 5% Senior Notes [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 [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	312,750	309,189
Senior Notes [Member] | Fair Value | 5% Senior Notes [Member]
Liabilities:
Long term debt fair value	510,130	510,055
Bank Borrowings [Member] | Carrying Amount
Liabilities:
Long term debt fair value	771,192	725,818
Bank Borrowings [Member] | Fair Value
Liabilities:
Long term debt fair value	$ 771,192	725,818
Other Liabilities [Member] | Interest Rate Swap [Member]
Liabilities:
Cash Flow Hedge Derivative Instrument Liabilities at Fair Value		$ 100