Derivative Instruments - Fair Value Amounts of Derivative Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	$ 15,431	$ 109,494
Gross Amounts Offset in the Balance Sheet	(10,073)	(1,039)
Net Amounts of Assets Presented in the Balance Sheet	5,358	108,455
Gross Amounts of Recognized Liabilities	(16,817)	(1,039)
Gross Amounts Offset in the Balance Sheet	10,073	1,039
Net Amounts of Liabilities Presented in the Balance Sheet	(6,744)	0
Derivative assets (current)
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	9,815	82,205
Gross Amounts Offset in the Balance Sheet	(8,396)	(1,039)
Net Amounts of Assets Presented in the Balance Sheet	1,419	81,166
Derivative assets (noncurrent)
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	5,616	27,289
Gross Amounts Offset in the Balance Sheet	(1,677)	0
Net Amounts of Assets Presented in the Balance Sheet	3,939	27,289
Derivative liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Liabilities	(14,246)	(1,039)
Gross Amounts Offset in the Balance Sheet	8,396	1,039
Net Amounts of Liabilities Presented in the Balance Sheet	(5,850)	0
Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Liabilities	(2,571)	0
Gross Amounts Offset in the Balance Sheet	1,677	0
Net Amounts of Liabilities Presented in the Balance Sheet	$ (894)	$ 0