Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value by Hierarchy Level (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	$ 119,837	$ 203,693
Other invested assets—at fair value	8,849	8,625
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	6,325,461	6,084,225
Derivative assets	954	1,913
Mortgage servicing rights	345
Total assets at fair value	6,446,597	6,289,831
Derivative Liabilities	1,262	1,509
Total liabilities at fair value	1,262	1,509
Level I | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	711,244	574,337
Derivative assets	0	0
Mortgage servicing rights	0
Total assets at fair value	724,844	660,038
Derivative Liabilities	0	0
Total liabilities at fair value	0	0
Level II | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	5,604,521	5,500,192
Derivative assets	954	1,912
Mortgage servicing rights	0
Total assets at fair value	5,711,712	5,620,096
Derivative Liabilities	397	817
Total liabilities at fair value	397	817
Level III | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	9,696	9,696
Derivative assets	0	1
Mortgage servicing rights	345
Total assets at fair value	10,041	9,697
Derivative Liabilities	865	692
Total liabilities at fair value	865	692
Loaned securities | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities		9,803
Loaned securities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	106,237	117,992
Loaned securities | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	13,600	75,898
Loaned securities | Level I | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities		9,803
Loaned securities | Level I | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	0	0
Loaned securities | Level I | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	13,600	75,898
Loaned securities | Level II | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities		0
Loaned securities | Level II | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	106,237	117,992
Loaned securities | Level II | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	0	0
Loaned securities | Level III | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities		0
Loaned securities | Level III | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	0	0
Loaned securities | Level III | Fair Value, Measurements, Recurring | Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loaned securities	0	0
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	5,209,025	5,188,228
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	137,858	134,593
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	156,765	142,785
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	2,503,736	2,511,905
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | RMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	1,049,473	1,014,071
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	523,580	558,383
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | CLO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	471,610	487,849
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Other ABS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	318,601	284,559
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value		5,087
Fixed-maturities available for sale | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	47,402	48,996
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	128,295	124,734
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	128,295	124,734
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | RMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | CLO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | Other ABS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value		0
Fixed-maturities available for sale | Level I | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	5,080,730	5,063,494
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	9,563	9,859
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	156,765	142,785
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	2,503,736	2,511,905
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | RMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	1,049,473	1,014,071
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	523,580	558,383
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | CLO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	471,610	487,849
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | Other ABS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	318,601	284,559
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | Foreign government and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value		5,087
Fixed-maturities available for sale | Level II | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	47,402	48,996
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | RMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | CLO
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | Other ABS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value		0
Fixed-maturities available for sale | Level III | Fair Value, Measurements, Recurring | Mortgage insurance-linked notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	103,256	106,423
Trading securities | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	68,457	70,844
Trading securities | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	24,335	24,913
Trading securities | Fair Value, Measurements, Recurring | RMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	5,665	5,930
Trading securities | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	4,799	4,736
Trading securities | Level I | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level I | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level I | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level I | Fair Value, Measurements, Recurring | RMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level I | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level II | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	103,256	106,423
Trading securities | Level II | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	68,457	70,844
Trading securities | Level II | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	24,335	24,913
Trading securities | Level II | Fair Value, Measurements, Recurring | RMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	5,665	5,930
Trading securities | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	4,799	4,736
Trading securities | Level III | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level III | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level III | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level III | Fair Value, Measurements, Recurring | RMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Trading securities | Level III | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Equity securities | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	155,405	89,057
Equity securities | Level I | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	148,297	81,170
Equity securities | Level II | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	4,608	5,387
Equity securities | Level III | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	2,500	2,500
Mortgage loans held for sale | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	147,034	32,755
Mortgage loans held for sale | Level I | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Mortgage loans held for sale | Level II | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	147,034	32,755
Mortgage loans held for sale | Level III | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Other investments | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	7,196	7,196
Other investments | Level I | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Other investments | Level II | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Other investments | Level III | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	7,196	7,196
Short-term investments | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	703,545	660,566
Short-term investments | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	2,600	2,700
Short-term investments | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	119,715	150,930
Short-term investments | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value		1,347
Short-term investments | Fair Value, Measurements, Recurring | Other ABS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	9,500	1,158
Short-term investments | Fair Value, Measurements, Recurring | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	137,078	135,998
Short-term investments | Fair Value, Measurements, Recurring | Money market instruments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	434,652	368,433
Short-term investments | Level I | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	434,652	368,433
Short-term investments | Level I | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level I | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level I | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value		0
Short-term investments | Level I | Fair Value, Measurements, Recurring | Other ABS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level I | Fair Value, Measurements, Recurring | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level I | Fair Value, Measurements, Recurring | Money market instruments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	434,652	368,433
Short-term investments | Level II | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	268,893	292,133
Short-term investments | Level II | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	2,600	2,700
Short-term investments | Level II | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	119,715	150,930
Short-term investments | Level II | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value		1,347
Short-term investments | Level II | Fair Value, Measurements, Recurring | Other ABS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	9,500	1,158
Short-term investments | Level II | Fair Value, Measurements, Recurring | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	137,078	135,998
Short-term investments | Level II | Fair Value, Measurements, Recurring | Money market instruments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | State and municipal obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | Corporate bonds and notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | CMBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value		0
Short-term investments | Level III | Fair Value, Measurements, Recurring | Other ABS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | Other investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Short-term investments | Level III | Fair Value, Measurements, Recurring | Money market instruments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments at fair value	0	0
Partnership investments | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets—at fair value	$ 2,000	$ 1,000