Debt (Schedule of Debt) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 11,377	$ 14,553
Long-term Debt, Excluding Current Maturities	39,385	39,260
Derivative, Amount of Hedged Item	299	315
Long-term debt	51,061	54,128
Secured Debt	37,001	43,129
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	2,329	4,274
Long-term Debt, Excluding Current Maturities	14,893	15,450
Derivative, Amount of Hedged Item	308	326
Long-term debt	17,530	20,050
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	9,048	10,279
Long-term Debt, Excluding Current Maturities	24,492	23,810
Derivative, Amount of Hedged Item	(9)	(11)
Long-term debt	33,531	34,078
Trust Preferred Securities Subject to Mandatory Redemption [Member]
Debt Instrument [Line Items]
Long-term Debt, Excluding Current Maturities	2,600	2,600
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Secured Debt	$ 6,100	$ 6,100