Fair Value of Financial Instruments - Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Note 5)	$ 95,286	$ 103,996
Other invested assets—at fair value	3,965	4,165
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	6,333,985	6,512,377
Embedded derivatives	1,792	4,200
Total assets at fair value	6,431,063	6,620,573
Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	538,891	738,480
Embedded derivatives	0	0
Total assets at fair value	571,549	776,482
Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,792,094	5,770,897
Embedded derivatives	0	0
Total assets at fair value	5,854,722	5,836,891
Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	3,000	3,000
Embedded derivatives	1,792	4,200
Total assets at fair value	4,792	7,200
Loaned securities, fair value | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Note 5)	62,628	65,994
Loaned securities, fair value | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Note 5)	32,658	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 (Note 5)	0	0
Loaned securities, fair value | Level I | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Note 5)	32,658	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 (Note 5)	62,628	65,994
Loaned securities, fair value | Level II | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities (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 (Note 5)	0	0
Loaned securities, fair value | Level III | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities (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	5,401,704	5,517,078
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	190,190	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	161,392	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	2,832,366	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	788,159	705,117
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	660,696	709,203
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	520,083	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	183,678	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	5,115	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	60,025	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	162,309	192,452
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	162,309	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	5,239,395	5,324,626
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | US government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	27,881	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	161,392	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	2,832,366	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	788,159	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	660,696	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	520,083	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	183,678	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	5,115	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	60,025	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 | US 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	218,018	256,546
Trading securities | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	86,349	94,637
Trading securities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	90,613	119,186
Trading securities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	8,601	9,438
Trading securities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	32,455	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	218,018	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	86,349	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	90,613	119,186
Trading securities | Level II | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	8,601	9,438
Trading securities | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	32,455	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	185,803	184,245
Equity securities | Level I | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	178,466	176,828
Equity securities | Level II | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	7,337	7,417
Equity securities | Level III | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	525,460	551,508
Short-term investments | Fair Value, Measurements, Recurring | US 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	12,170	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	127,729	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 | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	700
Short-term investments | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	198,116	274,535
Short-term investments | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	186,745	101,824
Short-term investments | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	198,116	369,200
Short-term investments | Level I | Fair Value, Measurements, Recurring | US 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 | Foreign government and agency securities
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	198,116	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	327,344	182,308
Short-term investments | Level II | Fair Value, Measurements, Recurring | US 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	12,170	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	127,729	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 | Foreign government and agency securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	700
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	186,745	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 | US 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 | Foreign 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 | 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	3,000	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	3,000	3,000
Partnership investments | Investments measured at NAV
Fair Value by Hierarchy Level [Line Items]
Other invested assets—at fair value	$ 1,000	$ 1,200