Securities Financing Activities - Summarized Securities Financing Agreements (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	$ 0	$ 0
Total	0	0
Repurchase agreements	0	0
Total	0	0
Counterparty And Cash Collateral Netting [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(2)	0
Total	(2)	0
Repurchase agreements	(356)	(494)
Total	(356)	(494)
Federal Agency CMOs [Member]
Securities Financing Transaction [Line Items]
Assets pledged as collateral	453
Liabilities pledged as collateral	356
Gross Amounts Presented In Balance Sheets [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	11	3
Total	11	3
Repurchase agreements	365	497
Total	365	497
Netting Adjustments [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(9)	(3)
Total	(9)	(3)
Repurchase agreements	(9)	(3)
Total	$ (9)	$ (3)