Indebtedness (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Subsidiary Debt
Debt Instrument [Line Items]
Face Value	$ 108,804,000
Carrying Value	108,805,000	$ 123,992,000
Repayments in 2019	6,000,000
Repayments in 2020	1,000,000
Repayments thereafter	102,000,000
Total senior notes and other debt
Debt Instrument [Line Items]
Face Value	1,435,307,000
Carrying Value	$ 1,427,575,000	1,882,028,000
Senior Notes \| Senior Notes due August 15, 2019
Debt Instrument [Line Items]
Interest Rate	6.15%
Face Value	$ 0
Carrying Value	$ 0	140,568,000
Senior Notes \| Senior Notes due September 15, 2019
Debt Instrument [Line Items]
Interest Rate	7.375%
Face Value	$ 0
Carrying Value	$ 0	299,816,000
Senior Notes \| Senior Notes due September 15, 2020
Debt Instrument [Line Items]
Interest Rate	5.375%
Face Value	$ 300,000,000
Carrying Value	$ 299,756,000	299,420,000
Senior Notes \| Senior Notes due January 1, 2022
Debt Instrument [Line Items]
Interest Rate	8.70%
Face Value	$ 76,503,000
Carrying Value	$ 76,343,000	76,273,000
Senior Notes \| Senior Notes due March 15, 2022
Debt Instrument [Line Items]
Interest Rate	4.625%
Face Value	$ 350,000,000
Carrying Value	$ 349,088,000	348,670,000
Senior Notes \| Senior Notes due February 15, 2037
Debt Instrument [Line Items]
Interest Rate	6.25%
Face Value	$ 250,000,000
Carrying Value	$ 248,116,000	248,006,000
Senior Notes \| Senior Notes due August 1, 2044
Debt Instrument [Line Items]
Interest Rate	4.75%
Face Value	$ 350,000,000
Carrying Value	345,467,000	345,283,000
Total subordinated debentures
Debt Instrument [Line Items]
Face Value	1,235,000,000
Carrying Value	$ 1,198,704,000	907,491,000
Total subordinated debentures \| Subordinated Debt due April 30,2053
Debt Instrument [Line Items]
Interest Rate	5.625%
Face Value	$ 350,000,000
Carrying Value	$ 341,356,000	341,097,000
Total subordinated debentures \| Subordinated Debt due March 1, 2056
Debt Instrument [Line Items]
Interest Rate	5.90%
Face Value	$ 110,000,000
Carrying Value	$ 106,262,000	106,159,000
Total subordinated debentures \| Subordinated Debt due June1, 2056
Debt Instrument [Line Items]
Interest Rate	5.75%
Face Value	$ 290,000,000
Carrying Value	$ 281,777,000	281,551,000
Total subordinated debentures \| Subordinated Debt due March 30, 2058
Debt Instrument [Line Items]
Interest Rate	5.70%
Face Value	$ 185,000,000
Carrying Value	$ 178,845,000	178,684,000
Total subordinated debentures \| Subordinated Debt due December 30, 2059
Debt Instrument [Line Items]
Interest Rate	5.10%
Face Value	$ 300,000,000
Carrying Value	$ 290,464,000	$ 0
Non-recourse loan
Debt Instrument [Line Items]
Interest Rate	4.21%
Face Value	$ 101,750,000

Face (par) amount of debt instrument at time of issuance.

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