Financial Derivatives (Derivative Assets and Liabilitites Offsetting) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Derivative Asset, Gross Fair Value	$ 1,903	$ 2,874
Derivative Asset, Fair Value Offset Amount	795	1,054
Derivative Asset, Cash Collateral	45	636
Derivative Asset, Net Fair Value	1,063	1,184
Offsetting Liabilities [Line Items]
Derivative Liability, Gross Fair Value	2,942	2,856
Derivative Liability Setoff Right Amount	795	1,054
Derivative Liability, Cash Collateral	648	763
Derivative Liability, Net Fair Value	1,499	1,039
Liability [Member]
Offsetting Liabilities [Line Items]
Derivative Liability, Gross Fair Value	2,942	2,856
Derivative Liability Setoff Right Amount	795	1,054
Derivative Liability, Cash Collateral	648	763
Derivative Liability, Net Fair Value	1,499	1,039
Derivative Liability, Net	1,499	1,039
Assets [Member]
Offsetting Assets [Line Items]
Derivative Asset, Gross Fair Value	1,903	2,874
Derivative Asset, Fair Value Offset Amount	795	1,054
Derivative Asset, Cash Collateral	45	636
Derivative Asset, Net Fair Value	1,063	1,184
Derivative Asset, Securities Collateral Held Under Master Netting Agreements	12	32
Derivative Asset, Net	1,051	1,152
Interest Rate Contracts [Member] | Liability [Member] | Exchange Cleared, Liability [Member]
Offsetting Liabilities [Line Items]
Derivative Liability, Gross Fair Value	18	260
Derivative Liability Setoff Right Amount		251
Derivative Liability, Cash Collateral
Derivative Liability, Net Fair Value	18	9
Derivative Liability, Net	18	9
Interest Rate Contracts [Member] | Liability [Member] | Over The Counter, Liability [Member]
Offsetting Liabilities [Line Items]
Derivative Liability, Gross Fair Value	1,996	1,703
Derivative Liability Setoff Right Amount	629	662
Derivative Liability, Cash Collateral	570	669
Derivative Liability, Net Fair Value	797	372
Derivative Liability, Net	797	372
Interest Rate Contracts [Member] | Assets [Member] | Exchange Cleared [Member]
Offsetting Assets [Line Items]
Derivative Asset, Gross Fair Value	24	827
Derivative Asset, Fair Value Offset Amount		251
Derivative Asset, Cash Collateral		567
Derivative Asset, Net Fair Value	24	9
Derivative Asset, Net	24	9
Interest Rate Contracts [Member] | Assets [Member] | Exchange Traded [Member]
Offsetting Assets [Line Items]
Derivative Asset, Gross Fair Value	2
Derivative Asset, Net Fair Value	2
Derivative Asset, Net	2
Interest Rate Contracts [Member] | Assets [Member] | Over the Counter [Member]
Offsetting Assets [Line Items]
Derivative Asset, Gross Fair Value	1,406	1,695
Derivative Asset, Fair Value Offset Amount	602	668
Derivative Asset, Cash Collateral	41	67
Derivative Asset, Net Fair Value	763	960
Derivative Asset, Securities Collateral Held Under Master Netting Agreements	12	32
Derivative Asset, Net	751	928
Foreign Exchange And Other Contract [Member] | Liability [Member]
Offsetting Liabilities [Line Items]
Derivative Liability, Gross Fair Value	928	893
Derivative Liability Setoff Right Amount	166	141
Derivative Liability, Cash Collateral	78	94
Derivative Liability, Net Fair Value	684	658
Derivative Liability, Net	684	658
Foreign Exchange And Other Contract [Member] | Assets [Member]
Offsetting Assets [Line Items]
Derivative Asset, Gross Fair Value	471	352
Derivative Asset, Fair Value Offset Amount	193	135
Derivative Asset, Cash Collateral	4	2
Derivative Asset, Net Fair Value	274	215
Derivative Asset, Net	$ 274	$ 215