Derivative Instruments and Other Hedging Activities - Narrative (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional amount of derivative liabilities	$ 1,643,393	$ 1,528,184
Cash as collateral for derivative transactions	(1,513)	(1,892)
Cash Posted As Variation Margin for Derivatives	2,700
Not Designated as Hedging Instruments
Derivative [Line Items]
Notional amount of derivative liabilities	1,534,893	1,419,684
Not Designated as Hedging Instruments | Other contracts
Derivative [Line Items]
Notional amount of derivative liabilities	$ 103,055	$ 106,518