Derivative Financial Instruments - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional value	$ 7,200,000,000
Net derivatives	33,000,000	$ 20,000,000
Cash collateral held relative to derivative exposure	0	0
Cash collateral pledged	32,000,000	$ 22,000,000
Chicago Mercantile Exchange
Derivative [Line Items]
Notional value	$ 6,600,000,000
Percent of total notional derivative contracts	91.60%
London Clearing House
Derivative [Line Items]
Notional value	$ 600,000,000
Percent of total notional derivative contracts	8.40%