Fair Value of Financial Instruments - Assets Measured at Fair Value by Hierarchy Level (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Note 5)	$ 79,073	$ 103,996
Other invested assets—at fair value	5,082	4,165
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,905,065	6,512,377
Embedded derivatives		4,200
Total assets at fair value	5,984,138	6,620,573
Embedded derivatives	6,495
Total liabilities at fair value	6,495
Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	440,040	738,480
Embedded derivatives		0
Total assets at fair value	503,737	776,482
Embedded derivatives	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	5,461,025	5,770,897
Embedded derivatives		0
Total assets at fair value	5,476,401	5,836,891
Embedded derivatives	0
Total liabilities at fair value	0
Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	4,000	3,000
Embedded derivatives		4,200
Total assets at fair value	4,000	7,200
Embedded derivatives	6,495
Total liabilities at fair value	6,495
Loaned securities, fair value | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Note 5)	15,376	65,994
Loaned securities, fair value | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Note 5)	63,697	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)	63,697	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)	15,376	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	5,228,744	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	180,504	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	154,980	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	2,757,786	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	776,386	705,117
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	618,603	709,203
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CLO
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	502,814	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	180,022	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	5,049	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	52,600	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	153,098	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	153,098	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	5,075,646	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	27,406	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	154,980	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	2,757,786	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	776,386	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	618,603	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	502,814	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	180,022	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	5,049	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	52,600	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	189,441	256,546
Trading securities | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	80,661	94,637
Trading securities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	78,289	119,186
Trading securities | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	7,941	9,438
Trading securities | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	22,550	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	189,441	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	80,661	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	78,289	119,186
Trading securities | Level II | Fair Value, Measurements, Recurring | RMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	7,941	9,438
Trading securities | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	22,550	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	146,574	184,245
Equity securities | Level I | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	137,601	176,828
Equity securities | Level II | Fair Value, Measurements, Recurring | Equity securities
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	8,973	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	336,306	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	12,070	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	105,571	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 | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	325
Short-term investments | Fair Value, Measurements, Recurring | Money market instruments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	149,341	274,535
Short-term investments | Fair Value, Measurements, Recurring | Other investments
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	68,999	101,824
Short-term investments | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	149,341	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 | Other ABS
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	149,341	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	186,965	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	12,070	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	105,571	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 | Other ABS
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	325
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	68,999	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 | Other ABS
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