Derivative Instruments and Other Hedging Activities - Narrative (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional amount of derivative liabilities	$ 1,761,381	$ 1,721,752
Cash as collateral for derivative transactions	0	(552)
Cash Posted As Variation Margin for Derivatives	37,300	5,100
Not Designated as Hedging Instruments
Derivative [Line Items]
Notional amount of derivative liabilities	1,761,381	1,613,252
Not Designated as Hedging Instruments | Other contracts
Derivative [Line Items]
Notional amount of derivative liabilities	$ 74,530	$ 86,744