Derivative Instruments and Other Hedging Activities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional amount of derivative liabilities	$ 3,313,072	$ 1,991,559
Cash posted as variation margin for derivatives	79,600	35,800
Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount of derivative liabilities	3,204,572	1,991,559
Derivatives not designated as hedging instruments | Other contracts
Derivative [Line Items]
Notional amount of derivative liabilities	$ 183,243	$ 85,623