Derivative Instruments and Other Hedging Activities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Derivative [Line Items]
Notional amount of derivative liabilities	$ 1,721,752	$ 1,991,559
Cash as collateral for derivative transactions	552
Cash Posted As Variation Margin for Derivatives	5,100	35,800
Not Designated as Hedging Instruments
Derivative [Line Items]
Notional amount of derivative liabilities	1,613,252	1,991,559
Not Designated as Hedging Instruments | Other contracts
Derivative [Line Items]
Notional amount of derivative liabilities	$ 86,744	$ 85,623