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Indebtedness (Details) - USD ($) | Dec. 31, 2023 | Dec. 31, 2022 | 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 | $ 5,686,000 |  |
Carrying Value | 5,686,000 | $ 6,478,000 |
Repayments thereafter | 6,000,000 |  |
Total senior notes and other debt |  |  |
Debt Instrument [Line Items] |  |  |
Face Value | 1,825,686,000 |  |
Carrying Value | $ 1,827,951,000 | 1,828,823,000 |
Senior Notes | Senior Notes due February 15, 2037 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 6.25% |  |
Face Value | $ 250,000,000 |  |
Carrying Value | $ 248,556,000 | 248,446,000 |
Senior Notes | Senior Notes due August 1, 2044 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.75% |  |
Face Value | $ 350,000,000 |  |
Carrying Value | $ 346,205,000 | 346,020,000 |
Senior Notes | Senior Notes Due May 12 2050 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.00% |  |
Face Value | $ 470,000,000 |  |
Carrying Value | $ 489,964,000 | 490,721,000 |
Senior Notes | Senior Notes Due September 30, 2061 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.15% |  |
Face Value | $ 350,000,000 |  |
Carrying Value | $ 343,129,000 | 342,945,000 |
Senior Notes | Senior Notes Due March 30, 2052 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 3.55% |  |
Face Value | $ 400,000,000 |  |
Carrying Value | 394,411,000 | 394,213,000 |
Total subordinated debentures |  |  |
Debt Instrument [Line Items] |  |  |
Face Value | 1,035,000,000 |  |
Carrying Value | $ 1,009,090,000 | 1,008,371,000 |
Total subordinated debentures | Subordinated Debt due March 30, 2058 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.70% |  |
Face Value | $ 185,000,000 |  |
Carrying Value | $ 179,489,000 | 179,328,000 |
Total subordinated debentures | Subordinated Debt due December 30, 2059 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 5.10% |  |
Face Value | $ 300,000,000 |  |
Carrying Value | $ 291,418,000 | 291,179,000 |
Total subordinated debentures | Subordinated Debt Due September 30 2060 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.25% |  |
Face Value | $ 250,000,000 |  |
Carrying Value | $ 244,668,000 | 244,523,000 |
Total subordinated debentures | Subordinated Debt Due March 30, 2061 |  |  |
Debt Instrument [Line Items] |  |  |
Interest Rate | 4.125% |  |
Face Value | $ 300,000,000 |  |
Carrying Value | $ 293,515,000 | $ 293,341,000 |
Line of Credit | Revolving Credit Facility |  |  |
Debt Instrument [Line Items] |  |  |
Unsecured borrowings |  |  | $ 300,000,000

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