Rollforward of Credit Losses Recognized in Earnings Related to Securities (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment		$ 0	$ 0	$ 0	$ 0
Available-for-sale Securities	[1]	31,444		31,444		$ 30,672
Ending balance		$ 22	$ 24	$ 22	$ 24

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $280 million and $0 million at September 30, 2017 and December 31, 2016, respectively.