Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt, due within one year	$ 9,578	$ 8,976
Long-term debt, due after one year	31,912	35,217
Total long-term debt	41,490	44,193
Trust preferred securities	2,600	2,600
Secured debt	33,781	39,596
Unsecured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	2,630	1,663
Long-term debt, due after one year	8,708	10,444
Total long-term debt	11,338	12,107
Secured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	6,948	7,313
Long-term debt, due after one year	23,204	24,773
Total long-term debt	30,152	32,086
Federal Home Loan Bank advances
Debt Instrument [Line Items]
Secured debt	$ 14,700	$ 14,900

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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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