Derivative Instruments and Other Hedging Activities - Narrative (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional amount of derivative liabilities	$ 1,729,766	$ 1,721,752
Cash as collateral for derivative transactions	0	(552)
Cash Posted As Variation Margin for Derivatives	28,800	5,100
Not Designated as Hedging Instruments
Derivative [Line Items]
Notional amount of derivative liabilities	1,729,766	1,613,252
Not Designated as Hedging Instruments | Other contracts
Derivative [Line Items]
Notional amount of derivative liabilities	$ 71,889	$ 86,744