Securities Financing Activities - Summarized Securities Financing Agreements (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
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	(6)	0
Total	(6)	0
Repurchase agreements	(283)	(370)
Total	(283)	(370)
Federal Agency CMOs [Member]
Securities Financing Transaction [Line Items]
Assets pledged as collateral	416
Liabilities pledged as collateral	283
Gross Amounts Presented In Balance Sheets [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	7	3
Total	7	3
Repurchase agreements	285	374
Total	285	374
Netting Adjustments [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(1)	(3)
Total	(1)	(3)
Repurchase agreements	(2)	(4)
Total	$ (2)	$ (4)