Debt (Narrative) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Debt Issuance Cost		$ 39,000,000	$ 37,000,000
Undrawn Outstanding Letters Of Credit In Favor Of Third Parties		126,000,000	54,000,000
Undrawn Outstanding Letters Of Credit Between Subsidiaries		1,100,000,000	$ 728,000,000
Line of Credit Facility, Maximum Borrowing Capacity		$ 950,000,000
Debt, Weighted Average Interest Rate		5.16%	5.09%
Long-term Debt, Gross		$ 5,081,000,000	$ 4,464,000,000
Long-term debt		5,042,000,000	4,427,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four		4,000,000
Line of Credit Facility, Commitment Fee Amount		10,000,000	6,000,000	$ 11,000,000
Subsidiary Committed Facility		$ 200,000,000
Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.95%
Debt Instrument, Face Amount		$ 400,000,000
Long-term Debt, Gross		$ 400,000,000	400,000,000
Subordinated Debentures Due 2056 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.75%
Debt Instrument, Face Amount		$ 400,000,000
Long-term Debt, Gross		$ 400,000,000	400,000,000
Junior Subordinated Debentures Due 2065 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.82%
Debt Instrument, Face Amount		$ 400,000,000
Long-term Debt, Gross		$ 319,000,000	319,000,000
Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.70%
Debt Instrument, Face Amount		$ 400,000,000
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.09%
Debt Instrument, Face Amount		$ 100,000,000
Long-term Debt, Gross		$ 70,000,000	74,000,000
Senior Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.90%
Debt Instrument, Face Amount		$ 600,000,000
Long-term Debt, Gross		$ 599,000,000	599,000,000
Senior Notes Due 2030
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.15%
Debt Instrument, Face Amount		$ 600,000,000
Long-term Debt, Gross		$ 598,000,000	598,000,000
Surplus Note Due 2051
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		4.00%
Debt Instrument, Face Amount		$ 500,000,000
Long-term Debt, Gross		$ 500,000,000	500,000,000
Surplus Note Due 2043
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		7.125%
Debt Instrument, Face Amount		$ 500,000,000
Long-term Debt, Gross		450,000,000	475,000,000
Debt Related Commitment Fees and Debt Issuance Costs		$ 6,000,000
Subordinated Debentures Due 2052
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		7.125%
Debt Instrument, Face Amount		$ 700,000,000
Long-term Debt, Gross		700,000,000	700,000,000
Syndicated Credit Facility $850 Million [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		850,000,000	850,000,000
Long-term Line of Credit	[1]	0	$ 0
Revolving Credit Facility
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,500,000,000

[1] Represents issued but undrawn letters of credit. There was no cash borrowed for the periods presented.