Indebtedness (Details) - USD ($)	1 Months Ended
	Jan. 31, 2021	Feb. 18, 2021	Dec. 31, 2020	Dec. 31, 2019
Subsidiary Debt
Debt Instrument [Line Items]
Face Value			$ 110,987,000
Carrying Value			110,987,000	$ 108,805,000
Repayments in 2019			3,000,000
Repayments in 2020			6,000,000
Repayments thereafter			102,000,000
Subordinated Debt due March 1, 2056 | Subsequent Event
Debt Instrument [Line Items]
Extinguishment of debt	$ 110,000,000
Total senior notes and other debt
Debt Instrument [Line Items]
Face Value			1,607,490,000
Carrying Value			$ 1,623,025,000	1,427,575,000
Senior Notes | Senior Notes Due September 15 2020
Debt Instrument [Line Items]
Interest Rate			5.375%
Face Value			$ 0
Carrying Value			$ 0	299,756,000
Senior Notes | Senior Notes due January 1, 2022
Debt Instrument [Line Items]
Interest Rate			8.70%
Face Value			$ 76,503,000
Carrying Value			$ 76,419,000	76,343,000
Senior Notes | Senior Notes due March 15, 2022
Debt Instrument [Line Items]
Interest Rate			4.625%
Face Value			$ 350,000,000
Carrying Value			$ 349,505,000	349,088,000
Senior Notes | Senior Notes due February 15, 2037
Debt Instrument [Line Items]
Interest Rate			6.25%
Face Value			$ 250,000,000
Carrying Value			$ 248,226,000	248,116,000
Senior Notes | Senior Notes due August 1, 2044
Debt Instrument [Line Items]
Interest Rate			4.75%
Face Value			$ 350,000,000
Carrying Value			$ 345,652,000	345,467,000
Senior Notes | Senior Notes Due May 12 2050
Debt Instrument [Line Items]
Interest Rate			4.00%
Face Value			$ 470,000,000
Carrying Value			492,236,000	0
Total subordinated debentures
Debt Instrument [Line Items]
Face Value			1,135,000,000
Carrying Value			$ 1,102,309,000	1,198,704,000
Total subordinated debentures | Subordinated Debt due April 30,2053
Debt Instrument [Line Items]
Interest Rate			5.625%
Face Value			$ 0
Carrying Value			$ 0	341,356,000
Total subordinated debentures | Subordinated Debt due March 1, 2056
Debt Instrument [Line Items]
Interest Rate			5.90%
Face Value			$ 110,000,000
Carrying Value			$ 106,365,000	106,262,000
Total subordinated debentures | Subordinated Debt due March 1, 2056 | Subsequent Event
Debt Instrument [Line Items]
Interest Rate	5.90%
Total subordinated debentures | Subordinated Debt due June1, 2056
Debt Instrument [Line Items]
Interest Rate			5.75%
Face Value			$ 290,000,000
Carrying Value			$ 282,003,000	281,777,000
Total subordinated debentures | Subordinated Debt due March 30, 2058
Debt Instrument [Line Items]
Interest Rate			5.70%
Face Value			$ 185,000,000
Carrying Value			$ 179,006,000	178,845,000
Total subordinated debentures | Subordinated Debt due December 30, 2059
Debt Instrument [Line Items]
Interest Rate			5.10%
Face Value			$ 300,000,000
Carrying Value			$ 290,702,000	290,464,000
Total subordinated debentures | Subordinated Debt Due September 30 2060
Debt Instrument [Line Items]
Interest Rate			4.25%
Face Value			$ 250,000,000
Carrying Value			$ 244,233,000	$ 0
Total subordinated debentures | Subordinated Debt Due 2061 | Subsequent Event
Debt Instrument [Line Items]
Interest Rate		4.125%
Face Value		$ 300,000,000
Non-recourse loan
Debt Instrument [Line Items]
Interest Rate			4.21%
Face Value			$ 101,750,000