Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Federal Funds Sold and Securities Borrowed or Purchased under Agreements to Resell [Member] | Assets [Member]
Offsetting Assets [Line Items]
Net amount	$ 467	$ 568
Gross amounts recognized	779	568
Gross amounts offset in the balance sheet	(312)	0
Net amounts presented in the balance sheet	467	568
Gross amounts not offset in the balance sheet	0	0
Cash Collateral Received/Pledged	0	0
Derivative Financial Instruments, Assets [Member] | Assets [Member]
Offsetting Assets [Line Items]
Net amount	28	47
Gross amounts recognized	43	64
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	43	64
Gross amounts not offset in the balance sheet	(15)	(17)
Cash Collateral Received/Pledged	0	0
Assets, Total [Member] | Assets [Member]
Offsetting Assets [Line Items]
Net amount	495	615
Gross amounts recognized	822	632
Gross amounts offset in the balance sheet	(312)	0
Net amounts presented in the balance sheet	510	632
Gross amounts not offset in the balance sheet	(15)	(17)
Cash Collateral Received/Pledged	0	0
Federal Funds Purchased and Securities Sold under Agreements to Repurchase [Member] | Other Liabilities [Member]
Offsetting Assets [Line Items]
Net amount	4,624	827
Gross amounts recognized	4,936	827
Gross amounts offset in the balance sheet	(312)	0
Net amounts presented in the balance sheet	4,624	827
Gross amounts not offset in the balance sheet	0	0
Cash Collateral Received/Pledged	0	0
Derivative Financial Instruments, Liabilities [Member] | Other Liabilities [Member]
Offsetting Assets [Line Items]
Net amount	9	25
Gross amounts recognized	35	59
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	35	59
Gross amounts not offset in the balance sheet	(15)	(17)
Cash Collateral Received/Pledged	(11)	(17)
Liabilities, Total [Member] | Other Liabilities [Member]
Offsetting Assets [Line Items]
Net amount	4,633	852
Gross amounts recognized	4,971	886
Gross amounts offset in the balance sheet	(312)	0
Net amounts presented in the balance sheet	4,659	886
Gross amounts not offset in the balance sheet	(15)	(17)
Cash Collateral Received/Pledged	$ (11)	$ (17)