Short-Term and Long-Term Debt - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Dec. 31, 2015	Oct. 31, 2013	Dec. 31, 2007	Dec. 31, 1998	Dec. 31, 1994
Debt Instrument [Line Items]
Long-term debt	$ 2,245,000,000
Maximum borrowing capacity	$ 3,835,000,000
Non-operating indebtedness of subsidiaries to total capitalization, maximum	7.50%
Five-year revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 2,000,000,000
Banking regulation, mortgage banking, net worth, minimum	$ 8,626,000,000
Percentage of aggregate net proceeds of equity issuances	50.00%
Debt to capital ratio (low end of range)	35.00%
Debt to capital ratio (high end of range)	100.00%
Surplus Note9.76 Due2024 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 0	$ 50,000,000										$ 50,000,000
Stated interest rate	9.76%
Surplus Note6.56 Due2028 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 500,000,000	500,000,000									$ 500,000,000
Stated interest rate	6.56%
Capital surplus repayment threshold	$ 2,300,000,000
Surplus Note111 Bps Due2028 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 71,000,000	71,000,000							$ 71,000,000
Surplus Note111 Bps Due2028 [Member] | Subordinated Debt | SOFR
Debt Instrument [Line Items]
Variable rate	1.11%
Surplus Note226 Bps Due2028 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 544,000,000	568,000,000
Debt instrument, face amount	287,000,000
Long-term debt	$ 544,000,000
Surplus Note226 Bps Due2028 [Member] | Subordinated Debt | SOFR
Debt Instrument [Line Items]
Variable rate	2.26%
Surplus Note6.03 Due2028 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 750,000,000	750,000,000									$ 750,000,000
Stated interest rate	6.03%
Capital surplus repayment threshold	$ 2,400,000,000
Surplus Note200 Bps Due2035 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 30,000,000	30,000,000						$ 30,000,000
Surplus Note200 Bps Due2035 [Member] | Subordinated Debt | SOFR
Debt Instrument [Line Items]
Variable rate	2.00%
Surplus Note155 Bps Due2037 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 25,000,000	25,000,000					$ 25,000,000
Surplus Note155 Bps Due2037 [Member] | Subordinated Debt | SOFR
Debt Instrument [Line Items]
Variable rate	1.55%
Surplus Note4.20 Due2037 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 50,000,000	50,000,000				$ 50,000,000
Stated interest rate	4.20%
Surplus Note100 Bps Due2037 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 154,000,000	154,000,000
Debt instrument, face amount										$ 375,000,000
Long-term debt	$ 154,000,000
Surplus Note100 Bps Due2037 [Member] | Subordinated Debt | SOFR
Debt Instrument [Line Items]
Variable rate	1.00%
Surplus Note4.50 Due2038 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 13,000,000	13,000,000			$ 13,000,000
Stated interest rate	4.50%
Surplus Notes4.225 Due2037 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 28,000,000	28,000,000		$ 28,000,000
Stated interest rate	4.225%
Surplus Notes4.00 Due2037 [Member] | Subordinated Debt
Debt Instrument [Line Items]
Surplus notes	$ 30,000,000	$ 30,000,000	$ 30,000,000
Stated interest rate			4.00%