Securities Financing Activities - Summarized Securities Financing Agreements (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
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	(370)	(494)
Total offsetting of financial liabilities	(370)	(494)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities Financing Transaction [Line Items]
Pledged Financial Instruments, Not Separately Reported, Securities	463
Liabilities associated with collateral pledged	370
Gross Amount Presented in Balance Sheet [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	3	3
Total offsetting of financial assets	3	3
Repurchase agreements	374	497
Total offsetting of financial liabilities	374	497
Netting Adjustments [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(3)	(3)
Total offsetting of financial assets	(3)	(3)
Repurchase agreements	(4)	(3)
Total offsetting of financial liabilities	$ (4)	$ (3)