Derivative Instruments and Other Hedging Activities Derivative Instruments and Other Hedging Activities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional amount of derivative liabilities	$ 2,480,018	$ 1,991,559
Cash Posted As Variation Margin for Derivatives	44,700	35,800
Not Designated as Hedging Instruments
Derivative [Line Items]
Notional amount of derivative liabilities	2,371,518	1,991,559
Not Designated as Hedging Instruments | Other contracts
Derivative [Line Items]
Notional amount of derivative liabilities	$ 98,858	$ 85,623