Fair Value Measurements - Reconciliation of the Beginning and Ending Balances for Derivative Instruments Classified as Level 3 in the Fair Value Hierarchy (Parenthetical) (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Fair Value, Inputs, Level 3
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Transfer out of Level 3	$ 2,947	[1]

[1] During first nine months 2018, we transferred $2.9 million of swaption contracts out of Level 3 due to the exercise of these swaptions by our counterparties.