Securities Financing Activities - Summarized Securities Financing Agreements (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
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	(1)
Total offsetting of financial assets	0	(1)
Repurchase agreements	(494)	(352)
Total offsetting of financial liabilities	(494)	(352)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities Financing Transaction [Line Items]
Pledged Financial Instruments, Not Separately Reported, Securities	602
Liabilities associated with collateral pledged	494
Gross Amount Presented in Balance Sheet [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	3	1
Total offsetting of financial assets	3	1
Repurchase agreements	497	352
Total offsetting of financial liabilities	497	352
Netting Adjustments [Member]
Securities Financing Transaction [Line Items]
Reverse repurchase agreements	(3)	0
Total offsetting of financial assets	(3)	0
Repurchase agreements	(3)	0
Total offsetting of financial liabilities	$ (3)	$ 0