Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 11,320	$ 10,981
Long-term Debt, Excluding Current Maturities	33,629	33,037
Derivative, Amount of Hedged Item	127	208
Long-term Debt	45,076	44,226
Trust preferred securities	2,600	2,600
Secured Debt	36,659	36,837
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	3,681	3,482
Long-term Debt, Excluding Current Maturities	11,153	11,909
Derivative, Amount of Hedged Item	190	240
Long-term Debt	15,024	15,631
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	7,639	7,499
Long-term Debt, Excluding Current Maturities	22,476	21,128
Derivative, Amount of Hedged Item	(63)	(32)
Long-term Debt	30,052	28,595
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Secured Debt	$ 12,700	$ 10,300