Derivative Financial Instruments - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional value	$ 6,500,000,000
Net derivatives	30,000,000	$ 20,000,000
Cash collateral held relative to derivative exposure	0	0
Cash collateral pledged	28,000,000	$ 22,000,000
Chicago Mercantile Exchange
Derivative [Line Items]
Notional value	$ 5,900,000,000
Percent of total notional derivative contracts	90.30%
London Clearing House
Derivative [Line Items]
Notional value	$ 600,000,000
Percent of total notional derivative contracts	9.70%