Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 63	$ 39
Derivative Liability, Fair Value, Gross Liability	65	41
Customer Securities for which Entity has Right to Sell or Repledge, Fair Value	11	2
Derivative assets in net asset position [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	62	38
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	62	38
Derivative Asset, Not Subject to Master Netting Arrangement	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative Asset	62	38
Derivative assets with no offsetting arrangements [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	1	1
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	1	1
Derivative Asset, Not Subject to Master Netting Arrangement	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative Asset	1	1
Total Derivative Assets [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	63	39
Gross Amounts Offset in the Condensed Consolidated Balance Sheet	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	63	39
Derivative Asset, Not Subject to Master Netting Arrangement	0	0
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative Asset	63	39
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	65	41
Derivative Liability, Fair Value, Amount Offset Against Collateral	65	41
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Derivative, Collateral, Right to Reclaim Cash	(1)	(1)
Derivative Liability	64	40
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	967	892
Derivative Liability, Fair Value, Amount Offset Against Collateral	967	892
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Derivative, Collateral, Right to Reclaim Cash	(967)	(892)
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	1,032	933
Derivative Liability, Fair Value, Amount Offset Against Collateral	1,032	933
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Derivative, Collateral, Right to Reclaim Cash	(968)	(893)
Derivative Liability	$ 64	$ 40