Derivative Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional value	$ 6,200,000,000
Net derivatives	30,000,000	$ 27,000,000
Cash collateral held relative to derivative exposure	0	0
Cash collateral pledged	32,000,000	$ 27,000,000
Chicago Mercantile Exchange
Derivative [Line Items]
Notional value	$ 5,600,000,000
Percent of total notional derivative contracts	90.90%
Amount of variation margin included as settlement	$ (33,200,000)
London Clearing House
Derivative [Line Items]
Notional value	$ 600,000,000
Percent of total notional derivative contracts	9.10%
Amount of variation margin included as settlement	$ 300,000