Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax Act Impacts [Line Items]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount									$ 527	$ 553	$ 488
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount									(49)	51	(25)
Effective Income Tax Rate Reconciliation, Tax Credit, Amount									(12)	(15)	(13)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount									7	35	38
Effective Income Tax Rate Reconciliation, Equity in Earnings (Losses) of Unconsolidated Subsidiary, Amount									4	7	14
Effective Income Tax Rate Reconciliation, Change in Unrecognized Tax Benefits									1	(161)	(5)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount									(16)	0	(1)
Income Tax Expense (Benefit)	$ 231	$ 115	$ 122	$ 113	$ 134	$ 130	$ 56	$ 150	581	470	496
Deferred Tax Assets, Net	461				$ 994				461	994
Income Tax Expense Before Tax Act Adjustments [Member]
Tax Act Impacts [Line Items]
Income Tax Expense (Benefit)									462	470	496
Net impacts as a result of the Tax Act [Member]
Tax Act Impacts [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									119	$ 0	$ 0
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount									549
Affordable Housing Tax Credits and Other Tax Benefits, Amount									7
Deferred Tax Assets, Net	421								421
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	$ 16								$ 16