Long-Term Debt - Components of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total long-term debt	$ 13,732	$ 14,333
Real Estate Loans and securities pledged	1,100	1,100
Senior Medium-Term Notes Due Through 2021 [Member]
Debt Instrument [Line Items]
Senior medium-term notes	$ 3,278	$ 2,766
Long-term debt weighted average interest rate	4.057%	3.56%
3.136% Subordinated notes due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.136%
Subordinated long-term notes	$ 162	$ 161
6.875% Subordinated Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.875%
Subordinated long-term notes	$ 104	109
7.750% Subordinated Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.75%
Subordinated long-term notes	$ 135	141
7.25% Subordinated notes due 2021 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.25%
Subordinated long-term notes	$ 336	$ 348
Long-term debt weighted average interest rate	7.25%	7.25%
6.75% Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.75%
Subordinated long-term notes	$ 315	$ 327
Long-term debt weighted average interest rate	6.75%	6.75%
Other Subordinated Notes [Member]
Debt Instrument [Line Items]
Subordinated long-term notes	$ 70	$ 69
Long-term debt weighted average interest rate	4.20%	3.022%
Senior Medium-Term Notes Due Through 2039 [Member]
Debt Instrument [Line Items]
Senior medium-term notes	$ 7,022	$ 8,011
Long-term debt weighted average interest rate	2.593%	2.24%
3.18% Senior Remarketable Notes Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.18%
Subordinated long-term notes	$ 212	$ 202
Long-term debt weighted average interest rate	3.18%	3.18%
4.625% Subordinated notes due 2018 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.625%
Subordinated long-term notes	$ 0	$ 100
3.40% Subordinated notes due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.40%
Subordinated long-term notes	$ 560	565
6.95% Subordinated notes due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.95%
Subordinated long-term notes	$ 299	299
Secured Borrowing Due Through 2025 [Member]
Debt Instrument [Line Items]
Secured borrowing due through 2025	$ 10	$ 24
Long-term debt weighted average interest rate	4.455%	4.46%
Federal Home Loan Bank Advances Due Through 2038 [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank advances due through 2038	$ 1,130	$ 1,106
Long-term debt weighted average interest rate	2.333%	2.318%
Investment Fund Financing Due Through 2052 [Member]
Debt Instrument [Line Items]
Investment Fund Financing due through 2052	$ 83	$ 88
Long-term debt weighted average interest rate	1.85%	1.94%
Obligations Under Capital Lease Due Through 2032 [Member]
Debt Instrument [Line Items]
Obligations under Capital Leases due 2032	$ 16	$ 17
Key [Member]
Debt Instrument [Line Items]
Total long-term debt	4,400	3,921
Subsidiaries [Member]
Debt Instrument [Line Items]
Total long-term debt	$ 9,332	$ 10,412

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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