Derivatives and Hedging Activities - Credit Derivatives Sold and Held (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Credit Derivatives [Line Items]
Notional Amount	$ 21		$ 15
Average Term (Years)	0 years
Payment / Performance Risk	0.00%		0.00%
Other [Member]
Credit Derivatives [Line Items]
Notional Amount	$ 21		$ 15
Average Term (Years)	10 years 4 months 17 days	3 years 29 days
Payment / Performance Risk	14.13%		6.64%