Indebtedness (Details) - USD ($)	Dec. 31, 2022	Apr. 01, 2022	Mar. 31, 2022	Jan. 31, 2022	Dec. 31, 2021
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	$ 6,478,000
Carrying Value	6,478,000				$ 10,564,000
Repayments in 2020	2,000,000
Repayments thereafter	5,000,000
Total senior notes and other debt
Debt Instrument [Line Items]
Face Value	1,826,478,000
Carrying Value	$ 1,828,823,000				2,259,416,000
Senior Notes | Senior Notes due January 1, 2022
Debt Instrument [Line Items]
Interest Rate	8.70%			8.70%
Face Value	$ 0			$ 77,000,000
Carrying Value	$ 0				76,503,000
Senior Notes | Senior Notes due March 15, 2022
Debt Instrument [Line Items]
Interest Rate	4.625%		4.625%
Face Value	$ 0		$ 350,000,000
Carrying Value	$ 0				349,923,000
Senior Notes | Senior Notes due February 15, 2037
Debt Instrument [Line Items]
Interest Rate	6.25%
Face Value	$ 250,000,000
Carrying Value	$ 248,446,000				248,336,000
Senior Notes | Senior Notes due August 1, 2044
Debt Instrument [Line Items]
Interest Rate	4.75%
Face Value	$ 350,000,000
Carrying Value	$ 346,020,000				345,836,000
Senior Notes | Senior Notes Due May 12 2050
Debt Instrument [Line Items]
Interest Rate	4.00%
Face Value	$ 470,000,000
Carrying Value	$ 490,721,000				491,478,000
Senior Notes | Senior Notes Due September 30, 2061
Debt Instrument [Line Items]
Interest Rate	3.15%
Face Value	$ 350,000,000
Carrying Value	$ 342,945,000				342,761,000
Senior Notes | Senior Notes Due March 30, 2052
Debt Instrument [Line Items]
Interest Rate	3.55%
Face Value	$ 400,000,000
Carrying Value	394,213,000				394,015,000
Total subordinated debentures
Debt Instrument [Line Items]
Face Value	1,035,000,000
Carrying Value	$ 1,008,371,000				1,007,652,000
Total subordinated debentures | Subordinated Debt due March 30, 2058
Debt Instrument [Line Items]
Interest Rate	5.70%
Face Value	$ 185,000,000
Carrying Value	$ 179,328,000				179,166,000
Total subordinated debentures | Subordinated Debt due December 30, 2059
Debt Instrument [Line Items]
Interest Rate	5.10%
Face Value	$ 300,000,000
Carrying Value	$ 291,179,000				290,941,000
Total subordinated debentures | Subordinated Debt Due September 30 2060
Debt Instrument [Line Items]
Interest Rate	4.25%
Face Value	$ 250,000,000
Carrying Value	$ 244,523,000				244,378,000
Total subordinated debentures | Subordinated Debt Due March 30, 2061
Debt Instrument [Line Items]
Interest Rate	4.125%
Face Value	$ 300,000,000
Carrying Value	$ 293,341,000				$ 293,167,000
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Unsecured borrowings		$ 300,000,000