Offsetting Assets and Liabilities Offsetting Assets and Liabilities (Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 39	$ 95
Derivative Liability, Fair Value, Gross Liability	41	95
Customer Securities for which Entity has Right to Sell or Repledge, Fair Value	2	6
Derivative assets in net asset position [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	38	87
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	38	87
Derivative Asset, Not Subject to Master Netting Arrangement	0	4
Derivative, Collateral, Obligation to Return Cash	0	(9)
Derivative Asset	38	74
Derivative assets in net liability positions [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset		8
Gross Amounts Offset in the Condensed Consolidated Balance Sheet		0
Derivative Asset, Fair Value, Amount Offset Against Collateral		8
Derivative Asset, Not Subject to Master Netting Arrangement		8
Derivative, Collateral, Obligation to Return Cash		0
Derivative Asset		0
Derivative assets with no offsetting arrangements [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	1
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1
Derivative Asset, Not Subject to Master Netting Arrangement	0
Derivative, Collateral, Obligation to Return Cash	0
Derivative Asset	1
Total Derivative Assets [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	39	95
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	39	95
Derivative Asset, Not Subject to Master Netting Arrangement	0	12
Derivative, Collateral, Obligation to Return Cash	0	(9)
Derivative Asset	39	74
Derivative Liabilities in Net Liability Positions [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Derivative Liability, Fair Value, Gross Liability	41	91
Derivative Liability, Fair Value, Amount Offset Against Collateral	41	91
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	8
Derivative, Collateral, Right to Reclaim Cash	1	13
Derivative Liability	40	70
Derivative liabilities in net asset positions [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Offset in the Condensed Consolidated Balance Sheet		0
Derivative Liability, Fair Value, Gross Liability		4
Derivative Liability, Fair Value, Amount Offset Against Collateral		4
Derivative Liability, Not Subject to Master Netting Arrangement Deduction		4
Derivative, Collateral, Right to Reclaim Cash		0
Derivative Liability		0
Total derivative liabilities excluding repurchase agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Offset in the Condensed Consolidated Balance Sheet		0
Derivative Liability, Fair Value, Gross Liability		95
Derivative Liability, Fair Value, Amount Offset Against Collateral		95
Derivative Liability, Not Subject to Master Netting Arrangement Deduction		12
Derivative, Collateral, Right to Reclaim Cash		13
Derivative Liability		70
Securities sold under agreement to repurchase [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Derivative Liability, Fair Value, Gross Liability	892	676
Derivative Liability, Fair Value, Amount Offset Against Collateral	892	676
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Derivative, Collateral, Right to Reclaim Cash	892	676
Derivative Liability	0	0
Total Derivative Liabilities [Member]
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Derivative Liability, Fair Value, Gross Liability	933	771
Derivative Liability, Fair Value, Amount Offset Against Collateral	933	771
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	12
Derivative, Collateral, Right to Reclaim Cash	893	689
Derivative Liability	$ 40	$ 70