Income Taxes Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Deferred Tax Assets, Tax Credit Carryforwards	$ 2,002
Deferred Tax Assets, Valuation Allowance	(1,123)
Deferred Tax Assets, Net	461	$ 994
Other Assets	5,663	6,854
Accounts Payable and Accrued Liabilities	1,780	1,737
Deferred Tax Asset [Member]
Tax Credit Carryforward [Line Items]
Deferred Tax Assets, Tax Credit Carryforwards	2,002	1,987
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Accrued Liabilities	450	546
Deferred Tax Assets, Tax Loss Carryforward	302	936
Deferred Tax Assets, State and local taxes	200	162
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Unearned Premiums Reserve	85	141
Deferred Tax Assets, Hedging Transactions	49	123
Deferred Tax Assets, Other	108	208
Deferred Tax Assets, Gross	3,196	4,103
Deferred Tax Assets, Valuation Allowance	(1,123)	(646)
Deferred Tax Assets, Net of Valuation Allowance	2,073	3,457
Other Assets	461	994
Deferred Tax Liability [Member]
Tax Credit Carryforward [Line Items]
Deferred Tax Liabilities, Leasing Arrangements	1,212	1,789
Deferred tax liabilities, deferred expense, deferred acquisition cost	269	424
Deferred Tax Liabilities, Financing Arrangements	95	161
Deferred Tax Liabilities, Other	44	107
Deferred Tax Liabilities, Gross	1,620	2,481
Deferred Tax Assets, Net	453	976
Accounts Payable and Accrued Liabilities	8	$ 18
Net impacts as a result of the Tax Act [Member]
Tax Credit Carryforward [Line Items]
Deferred Tax Assets, Net	421
Deferred Tax Assets, Change	128
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	549
Monetization of Deferred Tax Assets	$ 395