Fair Value Measurements - Reconciliation of the Beginning and Ending Balances for Derivative Instruments Classified as Level 3 in the Fair Value Hierarchy (Detail) - Level 3 $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2017	$ 6,658
Total losses included in earnings	(2,956)
Settlements	(1,781)
Transfer out of Level 3	(2,142)	[1]
Balance at June 30, 2018	$ (221)

[1] During first six months 2018, we transferred $2.1 million of swaption contracts out of Level 3 due to the exercise of these swaptions by our counterparties.