Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Available-for-sale securities:
Amortized cost	$ 1,401,504
Estimated fair value	1,387,147
Held-to-maturity securities - states & political subdivisions
Amortized cost	1,406,337	$ 1,368,061
Gross unrealized gains	9,347	17,455
Gross unrealized losses	23,789	15,825
Estimated fair value	1,391,895	1,369,691
Fair value of securities lent	55,300	64,500
Corporate debt securities
Available-for-sale securities:
Amortized cost	864,414	834,885
Gross unrealized gains	6,444	12,779
Gross unrealized losses	9,591	3,185
Estimated fair value	861,267	844,479
Collateralized debt obligations
Available-for-sale securities:
Amortized cost	135,831	133,224
Gross unrealized gains	99	207
Gross unrealized losses	535	164
Estimated fair value	135,395	133,267
Commercial mortgage-backed securities
Available-for-sale securities:
Amortized cost	144,079	139,516
Gross unrealized gains	1,934	2,808
Gross unrealized losses	1,895	1,783
Estimated fair value	144,118	140,541
Residential mortgage-backed securities
Available-for-sale securities:
Amortized cost	209,697	196,624
Gross unrealized gains	588	982
Gross unrealized losses	11,222	10,380
Estimated fair value	199,063	187,226
Other debt securities
Available-for-sale securities:
Amortized cost	35,908	34,863
Gross unrealized gains	271	543
Gross unrealized losses	359	254
Estimated fair value	35,820	35,152
U.S. Treasury
Available-for-sale securities:
Amortized cost	11,575	24,116
Gross unrealized gains	11	106
Gross unrealized losses	102	59
Estimated fair value	11,484	24,163
Total available-for-sale securities
Available-for-sale securities:
Amortized cost	1,401,504	1,363,228
Gross unrealized gains	9,347	17,425
Gross unrealized losses	23,704	15,825
Estimated fair value	1,387,147	1,364,828
Held-to-maturity securities - states & political subdivisions
Fair value of securities lent	37,000	44,400
Held-to-maturity securities
Held-to-maturity securities - states & political subdivisions
Amortized cost	4,833	4,833
Gross unrealized gains	0	30
Gross unrealized losses	85	0
Estimated fair value	$ 4,748	$ 4,863