Derivatives and Hedging Activities - Credit Derivatives Sold and Held (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Credit Derivatives [Line Items]
Notional Amount	$ 12		$ 15
Average Term (Years)	0 years
Payment / Performance Risk	0.00%		0.00%
Other [Member]
Credit Derivatives [Line Items]
Notional Amount	$ 12		$ 15
Average Term (Years)	2 years 5 months 19 days	3 years 29 days
Payment / Performance Risk	5.01%		6.64%