Derivative Instruments - Fair Value Amounts of Derivative Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	$ 47,241	$ 109,494
Gross Amounts Offset in the Balance Sheet	(1,117)	(1,039)
Net Amounts of Assets Presented in the Balance Sheet	46,124	108,455
Gross Amounts of Recognized Liabilities	(1,117)	(1,039)
Gross Amounts Offset in the Balance Sheet	1,117	1,039
Net Amounts of Liabilities Presented in the Balance Sheet	0	0
Derivative assets (current)
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	37,218	82,205
Gross Amounts Offset in the Balance Sheet	(1,104)	(1,039)
Net Amounts of Assets Presented in the Balance Sheet	36,114	81,166
Derivative assets (noncurrent)
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	10,023	27,289
Gross Amounts Offset in the Balance Sheet	(13)	0
Net Amounts of Assets Presented in the Balance Sheet	10,010	27,289
Accounts payable and accrued liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Liabilities	(1,104)	(1,039)
Gross Amounts Offset in the Balance Sheet	1,104	1,039
Net Amounts of Liabilities Presented in the Balance Sheet	0	0
Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Liabilities	(13)	0
Gross Amounts Offset in the Balance Sheet	13	0
Net Amounts of Liabilities Presented in the Balance Sheet	$ 0	$ 0