Derivative Instruments - Fair Value of Derivative Instruments on a Gross basis and on a Net Basis as Presented in Consolidated Balance Sheets (Detail) - Commodity Contract [Member] - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Gross Amount		$ 50	$ 45,793
Netting Adjustments	[1]	(50)	(11,352)
Net Amount Presented in Balance Sheets			34,441
Gross Amount		(48,132)	(11,352)
Netting Adjustments	[1]	50	11,352
Net Amount Presented in Balance Sheets		(48,082)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		50	41,199
Netting Adjustments	[1]	(50)	(8,158)
Net Amount Presented in Balance Sheets			33,041
Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amount			4,594
Netting Adjustments	[1]		(3,194)
Net Amount Presented in Balance Sheets			1,400
Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount			(8,158)
Netting Adjustments	[1]		8,158
Current Liabilities [Member] | Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(35,459)
Netting Adjustments	[1]	50
Net Amount Presented in Balance Sheets		(35,409)
Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amount		(12,673)	(3,194)
Netting Adjustments	[1]		$ 3,194
Net Amount Presented in Balance Sheets		$ (12,673)

[1] The Company has agreements in place that allow for the financial right to offset for derivative assets and derivative liabilities at settlement or in the event of a default under the agreements.