Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 10,756	$ 10,981
Long-term Debt, Excluding Current Maturities	36,473	33,037
Long-term Debt	47,328	44,226
Trust preferred securities	2,600	2,600
Secured Debt	37,723	36,837
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	2,740	3,482
Long-term Debt, Excluding Current Maturities	11,139	11,909
Long-term Debt	14,047	15,631
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	8,016	7,499
Long-term Debt, Excluding Current Maturities	25,334	21,128
Long-term Debt	33,281	28,595
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Secured Debt	14,700	10,300
Fair Value Hedging [Member]
Debt Instrument [Line Items]
Long-term Debt, Fair Value Hedge Basis Adjustment	99	208
Long-term Debt	99	208
Fair Value Hedging [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Fair Value Hedge Basis Adjustment	168	240
Fair Value Hedging [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Fair Value Hedge Basis Adjustment	$ (69)	$ (32)