Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt, due within one year	$ 9,662	$ 10,981
Long-term debt, due after one year	35,818	33,037
Total long-term debt	45,542	44,226
Trust preferred securities	2,600	2,600
Secured debt	36,992	36,837
Unsecured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	2,043	3,482
Long-term debt, due after one year	11,135	11,909
Total long-term debt	13,313	15,631
Secured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	7,619	7,499
Long-term debt, due after one year	24,683	21,128
Total long-term debt	32,229	28,595
Federal Home Loan Bank advances
Debt Instrument [Line Items]
Secured debt	14,000	10,300
Fair value hedging
Debt Instrument [Line Items]
Long-term debt, fair value adjustment	62	208
Total long-term debt	62	208
Fair value hedging \| Unsecured debt
Debt Instrument [Line Items]
Long-term debt, fair value adjustment	135	240
Fair value hedging \| Secured debt
Debt Instrument [Line Items]
Long-term debt, fair value adjustment	$ (73)	$ (32)

Long-term Debt, Fair Value Hedge Basis Adjustment

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Trust preferred securities

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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