Fair Value of Financial Instruments - Fair Value Assets Liabilities by Hierarchy Level (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	$ 103,996	$ 57,499
Other investments	4,165	4,973
Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	6,512,377	6,786,469
Embedded derivatives	4,200	5,513
Total assets at fair value	6,620,573	6,849,481
Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,517,078	5,723,340
Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	256,546	290,885
Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	184,245	151,240
Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	551,508	618,004
Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	3,000	3,000
Partnership investment | Carrying Amount
Fair Value by Hierarchy Level [Line Items]
Other investments	1,200	2,000
Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	738,480	551,695
Embedded derivatives	0	0
Total assets at fair value	776,482	577,870
Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	192,452	140,034
Level I | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Level I | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	176,828	142,761
Level I | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	369,200	268,900
Level I | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,770,897	6,231,774
Embedded derivatives	0	0
Total assets at fair value	5,836,891	6,263,098
Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,324,626	5,583,306
Level II | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	256,546	290,885
Level II | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	7,417	8,479
Level II | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	182,308	349,104
Level II | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	3,000	3,000
Embedded derivatives	4,200	5,513
Total assets at fair value	7,200	8,513
Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Level III | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Level III | Equity securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Level III | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Level III | Other investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	3,000	3,000
Loaned securities | Fixed-maturities available for sale
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	65,600
Loaned securities | Equity securities
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	38,000
Loaned securities | Short-term investments
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	400
U.S. government and agency securities | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	221,730	169,223
U.S. government and agency securities | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	94,665
U.S. government and agency securities | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	192,452	140,034
U.S. government and agency securities | Level I | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	94,665
U.S. government and agency securities | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	29,278	29,189
U.S. government and agency securities | Level II | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0
U.S. government and agency securities | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
U.S. government and agency securities | Level III | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0
U.S. government and agency securities | Loaned securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)		4,876
U.S. government and agency securities | Loaned securities | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)		4,876
U.S. government and agency securities | Loaned securities | Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)		0
U.S. government and agency securities | Loaned securities | Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)		0
State and municipal obligations | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	177,257	165,271
State and municipal obligations | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	94,637	120,449
State and municipal obligations | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	12,270	21,819
State and municipal obligations | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
State and municipal obligations | Level I | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
State and municipal obligations | Level I | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
State and municipal obligations | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	177,257	165,271
State and municipal obligations | Level II | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	94,637	120,449
State and municipal obligations | Level II | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	12,270	21,819
State and municipal obligations | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
State and municipal obligations | Level III | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
State and municipal obligations | Level III | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Money market instruments | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	274,535	268,900
Money market instruments | Level I | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	274,535	268,900
Money market instruments | Level II | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Money market instruments | Level III | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Corporate bonds and notes | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	2,910,231	3,047,189
Corporate bonds and notes | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	119,186	123,142
Corporate bonds and notes | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	65,191	30,495
Corporate bonds and notes | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Corporate bonds and notes | Level I | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Corporate bonds and notes | Level I | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Corporate bonds and notes | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	2,910,231	3,047,189
Corporate bonds and notes | Level II | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	119,186	123,142
Corporate bonds and notes | Level II | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	65,191	30,495
Corporate bonds and notes | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Corporate bonds and notes | Level III | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Corporate bonds and notes | Level III | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Corporate bonds and notes | Loaned securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	65,994	31,324
Corporate bonds and notes | Loaned securities | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	0	0
Corporate bonds and notes | Loaned securities | Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	65,994	31,324
Corporate bonds and notes | Loaned securities | Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	0	0
RMBS | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	705,117	833,939
RMBS | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	9,438	13,000
RMBS | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
RMBS | Level I | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
RMBS | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	705,117	833,939
RMBS | Level II | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	9,438	13,000
RMBS | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
RMBS | Level III | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
CMBS | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	709,203	681,265
CMBS | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	33,285	34,294
CMBS | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	3,023
CMBS | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
CMBS | Level I | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
CMBS | Level I | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0
CMBS | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	709,203	681,265
CMBS | Level II | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	33,285	34,294
CMBS | Level II | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	3,023
CMBS | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
CMBS | Level III | Trading securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
CMBS | Level III | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0
CLO | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	530,040	568,558
CLO | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
CLO | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	530,040	568,558
CLO | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Other ABS | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	211,187	252,457
Other ABS | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		219
Other ABS | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Other ABS | Level I | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		0
Other ABS | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	211,187	252,457
Other ABS | Level II | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		219
Other ABS | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Other ABS | Level III | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value		0
Foreign government and agency securities | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,296	5,438
Foreign government and agency securities | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Foreign government and agency securities | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	5,296	5,438
Foreign government and agency securities | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Mortgage insurance-linked notes | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	47,017
Mortgage insurance-linked notes | Level I | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0
Mortgage insurance-linked notes | Level II | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	47,017
Mortgage insurance-linked notes | Level III | Fixed-maturities available for sale | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0
Equity securities | Loaned securities | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	38,002	21,299
Equity securities | Loaned securities | Level I | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	38,002	21,299
Equity securities | Loaned securities | Level II | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	0	0
Equity securities | Loaned securities | Level III | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Loaned securities (Notes 5 and 6)	0	0
Other investments | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	101,824	296,571
Other investments | Level I | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	0	0
Other investments | Level II | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	101,824	296,571
Other investments | Level III | Short-term investments | Fair Value, Measurements, Recurring
Fair Value by Hierarchy Level [Line Items]
Total investments at fair value	$ 0	$ 0