Capital Structure - Interest rates for LIBOR-linked debt (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2023	Dec. 31, 2022
Debt outstanding:
Debt outstanding	$ 7,949	$ 7,964
5.100% Subordinated Debentures, due 2053
Debt outstanding:
Interest rate	5.10%
Debt outstanding	$ 500
5.100% Subordinated Debentures, due 2053 | Secured Overnight Financing Rate (SOFR) | Minimum
Debt outstanding:
Basis spread on interest at floating rate	0.26161%
5.100% Subordinated Debentures, due 2053 | Secured Overnight Financing Rate (SOFR) | Maximum
Debt outstanding:
Basis spread on interest at floating rate	3.165%
5.750% Subordinated Debentures, due 2053
Debt outstanding:
Interest rate	5.75%
Debt outstanding	$ 800
5.750% Subordinated Debentures, due 2053 | Secured Overnight Financing Rate (SOFR) | Minimum
Debt outstanding:
Basis spread on interest at floating rate	0.26161%
5.750% Subordinated Debentures, due 2053 | Secured Overnight Financing Rate (SOFR) | Maximum
Debt outstanding:
Basis spread on interest at floating rate	2.938%