Indebtedness (Details) - USD ($)	Dec. 31, 2024	Dec. 31, 2023	Apr. 01, 2022
Debt Instrument [Line Items]
Unsecured borrowings, limit increase			$ 500,000,000
Standby Letters of Credit
Debt Instrument [Line Items]
Unsecured borrowings			50,000,000
Subsidiary Debt
Debt Instrument [Line Items]
Face Value	$ 8,973,000
Carrying Value	8,973,000	$ 5,686,000
Repayments thereafter	9,400,000
Total senior notes and other debt
Debt Instrument [Line Items]
Face Value	1,828,973,000
Carrying Value	$ 1,831,158,000	1,827,951,000
Senior Notes | Senior Notes due February 15, 2037
Debt Instrument [Line Items]
Interest Rate	6.25%
Face Value	$ 250,000,000
Carrying Value	$ 248,666,000	248,556,000
Senior Notes | Senior Notes due August 1, 2044
Debt Instrument [Line Items]
Interest Rate	4.75%
Face Value	$ 350,000,000
Carrying Value	$ 346,389,000	346,205,000
Senior Notes | Senior Notes Due May 12 2050
Debt Instrument [Line Items]
Interest Rate	4.00%
Face Value	$ 470,000,000
Carrying Value	$ 489,207,000	489,964,000
Senior Notes | Senior Notes Due September 30, 2061
Debt Instrument [Line Items]
Interest Rate	3.15%
Face Value	$ 350,000,000
Carrying Value	$ 343,314,000	343,129,000
Senior Notes | Senior Notes Due March 30, 2052
Debt Instrument [Line Items]
Interest Rate	3.55%
Face Value	$ 400,000,000
Carrying Value	394,609,000	394,411,000
Total subordinated debentures
Debt Instrument [Line Items]
Face Value	1,035,000,000
Carrying Value	$ 1,009,808,000	1,009,090,000
Total subordinated debentures | Subordinated Debt due March 30, 2058
Debt Instrument [Line Items]
Interest Rate	5.70%
Face Value	$ 185,000,000
Carrying Value	$ 179,650,000	179,489,000
Total subordinated debentures | Subordinated Debt due December 30, 2059
Debt Instrument [Line Items]
Interest Rate	5.10%
Face Value	$ 300,000,000
Carrying Value	$ 291,656,000	291,418,000
Total subordinated debentures | Subordinated Debt Due September 30 2060
Debt Instrument [Line Items]
Interest Rate	4.25%
Face Value	$ 250,000,000
Carrying Value	$ 244,813,000	244,668,000
Total subordinated debentures | Subordinated Debt Due March 30, 2061
Debt Instrument [Line Items]
Interest Rate	4.125%
Face Value	$ 300,000,000
Carrying Value	$ 293,689,000	$ 293,515,000
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Unsecured borrowings			$ 300,000,000