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	3 Months Ended
	Mar. 31, 2018 USD ($)
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2017	$ 6,658
Total gain (losses) included in earnings	2,857
Settlements	2,799
Transfer out of Level 3	(2,142)	[1]
Balance at March 31, 2018	$ 10,172

[1] During first quarter 2018, we transferred $2.1 million of swaption contracts out of Level 3 resulting from the exercise of our Level 3 swaptions by the counterparties.