Fair Value of Financial Instruments - Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	$ 137,282	$ 103,996
Other invested assets—at fair value	5,079	4,165
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,590,802	6,512,377
Derivative assets	707	4,200
Total assets at fair value	5,728,791	6,620,573
Derivative liabilities	4,634
Total liabilities at fair value	4,634
Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	519,655	738,480
Derivative assets	0	0
Total assets at fair value	580,021	776,482
Derivative liabilities	0
Total liabilities at fair value	0
Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,064,647	5,770,897
Derivative assets	56	0
Total assets at fair value	5,141,619	5,836,891
Derivative liabilities	33
Total liabilities at fair value	33
Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	6,500	3,000
Derivative assets	651	4,200
Total assets at fair value	7,151	7,200
Derivative liabilities	4,601
Total liabilities at fair value	4,601
Loaned securities, fair value
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	76,174
Loaned securities, fair value | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	40,377	65,994
Loaned securities, fair value | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	9,651
Loaned securities, fair value | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	7,117
Loaned securities, fair value | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	2,876
Loaned securities, fair value | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	16,895
Loaned securities, fair value | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	60,366	38,002
Loaned securities, fair value | Level I | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0	0
Loaned securities, fair value | Level I | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0
Loaned securities, fair value | Level I | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0
Loaned securities, fair value | Level I | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0
Loaned securities, fair value | Level I | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0
Loaned securities, fair value | Level I | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	60,366	38,002
Loaned securities, fair value | Level II | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	40,377	65,994
Loaned securities, fair value | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	9,651
Loaned securities, fair value | Level II | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	7,117
Loaned securities, fair value | Level II | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	2,876
Loaned securities, fair value | Level II | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	16,895
Loaned securities, fair value | Level II | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0	0
Loaned securities, fair value | Level III | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0	0
Loaned securities, fair value | Level III | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0
Loaned securities, fair value | Level III | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0
Loaned securities, fair value | Level III | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0
Loaned securities, fair value | Level III | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0
Loaned securities, fair value | Level III | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities and securities sold under repurchase agreements (Note 5)	0	0
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	4,909,232	5,517,078
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	146,664	221,730
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	142,743	177,257
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	2,429,325	2,910,231
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	900,591	705,117
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	599,478	709,203
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	490,832	530,040
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	168,087	211,187
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	4,970	5,296
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	26,542	47,017
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	142,882	192,452
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	142,882	192,452
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	4,766,350	5,324,626
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	3,782	29,278
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	142,743	177,257
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	2,429,325	2,910,231
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	900,591	705,117
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	599,478	709,203
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	490,832	530,040
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	168,087	211,187
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	4,970	5,296
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	26,542	47,017
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	139,901	256,546
Trading securities | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	70,475	94,637
Trading securities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	47,802	119,186
Trading securities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	7,058	9,438
Trading securities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	14,566	33,285
Trading securities | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level I | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level I | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level I | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level I | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	139,901	256,546
Trading securities | Level II | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	70,475	94,637
Trading securities | Level II | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	47,802	119,186
Trading securities | Level II | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	7,058	9,438
Trading securities | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	14,566	33,285
Trading securities | Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level III | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level III | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level III | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Trading securities | Level III | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Equity securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	145,155	184,245
Equity securities | Level I | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	134,907	176,828
Equity securities | Level II | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	7,748	7,417
Equity securities | Level III | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	2,500	0
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	392,514	551,508
Short-term investments | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		94,665
Short-term investments | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	2,885	12,270
Short-term investments | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	53,406	65,191
Short-term investments | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		3,023
Short-term investments | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	241,866	274,535
Short-term investments | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	94,357	101,824
Short-term investments | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	241,866	369,200
Short-term investments | Level I | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		94,665
Short-term investments | Level I | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Level I | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Level I | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		0
Short-term investments | Level I | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	241,866	274,535
Short-term investments | Level I | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	150,648	182,308
Short-term investments | Level II | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		0
Short-term investments | Level II | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	2,885	12,270
Short-term investments | Level II | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	53,406	65,191
Short-term investments | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		3,023
Short-term investments | Level II | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Level II | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	94,357	101,824
Short-term investments | Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		0
Short-term investments | Level III | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		0
Short-term investments | Level III | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	4,000	3,000
Other investments | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Other investments | Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Other investments | Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	4,000	3,000
Partnership investments | Investments measured at NAV
Fair Value by Hierarchy Level [Line Items]
Other invested assets—at fair value	$ 1,100	$ 1,200