Derivative Instruments and Other Hedging Activities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Mar. 31, 2017
Derivative [Line Items]
Notional amount of derivative liabilities	$ 1,528,184	$ 1,729,550
Cash as collateral for derivative transactions	1,900
Cash Posted As Variation Margin for Derivatives		3,900
Not Designated as Hedging Instruments
Derivative [Line Items]
Notional amount of derivative liabilities	1,419,684	1,621,050
Not Designated as Hedging Instruments | Other contracts
Derivative [Line Items]
Notional amount of derivative liabilities	$ 106,518	$ 110,485