Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Long-term debt, due within one year	$ 5,637	$ 5,085
Long-term debt, due after one year	14,866	16,921
Total long-term debt	20,503	22,006
Trust preferred securities	2,600	2,600
Secured Debt	8,701	9,992
Unsecured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	1,219	647
Long-term debt, due after one year	10,583	11,367
Total long-term debt	11,802	12,014
Secured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	4,418	4,438
Long-term debt, due after one year	4,283	5,554
Total long-term debt	8,701	9,992
Federal Home Loan Bank advances
Debt Instrument [Line Items]
Secured Debt	$ 5,100	$ 5,800

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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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