Derivative Instruments and Other Hedging Activities - Narrative (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional amount of derivative liabilities	$ 1,617,136	$ 1,528,184
Cash as collateral for derivative transactions	(983)	(1,892)
Cash Posted As Variation Margin for Derivatives	2,600
Not Designated as Hedging Instruments
Derivative [Line Items]
Notional amount of derivative liabilities	1,508,636	1,419,684
Not Designated as Hedging Instruments | Other contracts
Derivative [Line Items]
Notional amount of derivative liabilities	$ 88,995	$ 106,518