Securities Financing Activities - Summarized Securities Financing Agreements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	$ 0	$ 0
Total offsetting of financial assets	0	0
Repurchase agreements	0	0
Total offsetting of financial liabilities	0	0
Counterparty And Cash Collateral Netting [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	0	0
Total offsetting of financial assets	0	0
Repurchase agreements	(305)	(370)
Total offsetting of financial liabilities	(305)	(370)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities Financing Transaction [Line Items]
Carrying amount of assets pledged as collateral against repurchase agreements	892
Liabilities associated with collateral pledged	304
Gross Amount Presented in Balance Sheet [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	14	3
Total offsetting of financial assets	14	3
Repurchase agreements	319	374
Total offsetting of financial liabilities	319	374
Netting Adjustments [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(14)	(3)
Total offsetting of financial assets	(14)	(3)
Repurchase agreements	(14)	(4)
Total offsetting of financial liabilities	$ (14)	$ (4)