Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Federal Funds Sold and Securities Borrowed or Purchased under Agreements to Resell [Member] | Assets [Member]
Offsetting Assets [Line Items]
Net amount	$ 696	$ 514
Gross amounts recognized	793	809
Gross amounts offset in the balance sheet	(97)	(295)
Net amounts presented in the balance sheet	696	514
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	18	28
Gross amounts recognized	28	38
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	28	38
Gross amounts not offset in the balance sheet	(5)	(9)
Cash Collateral Received/Pledged	(5)	(1)
Assets, Total [Member] | Assets [Member]
Offsetting Assets [Line Items]
Net amount	714	542
Gross amounts recognized	821	847
Gross amounts offset in the balance sheet	(97)	(295)
Net amounts presented in the balance sheet	724	552
Gross amounts not offset in the balance sheet	(5)	(9)
Cash Collateral Received/Pledged	(5)	(1)
Federal Funds Purchased and Securities Sold under Agreements to Repurchase [Member] | Other Liabilities [Member]
Offsetting Assets [Line Items]
Net amount	4,867	4,976
Gross amounts recognized	4,964	5,271
Gross amounts offset in the balance sheet	(97)	(295)
Net amounts presented in the balance sheet	4,867	4,976
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	49	25
Gross amounts recognized	56	40
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	56	40
Gross amounts not offset in the balance sheet	(5)	(9)
Cash Collateral Received/Pledged	(2)	(6)
Liabilities, Total [Member] | Other Liabilities [Member]
Offsetting Assets [Line Items]
Net amount	4,916	5,001
Gross amounts recognized	5,020	5,311
Gross amounts offset in the balance sheet	(97)	(295)
Net amounts presented in the balance sheet	4,923	5,016
Gross amounts not offset in the balance sheet	(5)	(9)
Cash Collateral Received/Pledged	$ (2)	$ (6)