Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Available-for-sale securities:
Amortized cost	$ 1,446,197
Estimated fair value	1,433,222
Held-to-maturity securities - states & political subdivisions
Amortized cost	1,451,030	$ 1,368,061
Gross unrealized gains	6,531	17,455
Gross unrealized losses	19,491	15,825
Estimated fair value	1,438,070	1,369,691
Fair value of securities lent	69,900	64,500
Corporate debt securities
Available-for-sale securities:
Amortized cost	784,968	834,885
Gross unrealized gains	4,200	12,779
Gross unrealized losses	4,794	3,185
Estimated fair value	784,374	844,479
Collateralized debt obligations
Available-for-sale securities:
Amortized cost	233,053	133,224
Gross unrealized gains	155	207
Gross unrealized losses	487	164
Estimated fair value	232,721	133,267
Commercial mortgage-backed securities
Available-for-sale securities:
Amortized cost	147,887	139,516
Gross unrealized gains	1,550	2,808
Gross unrealized losses	1,715	1,783
Estimated fair value	147,722	140,541
Residential mortgage-backed securities
Available-for-sale securities:
Amortized cost	220,319	196,624
Gross unrealized gains	421	982
Gross unrealized losses	12,055	10,380
Estimated fair value	208,685	187,226
Other debt securities
Available-for-sale securities:
Amortized cost	46,032	34,863
Gross unrealized gains	187	543
Gross unrealized losses	385	254
Estimated fair value	45,834	35,152
U.S. Treasury
Available-for-sale securities:
Amortized cost	13,938	24,116
Gross unrealized gains	3	106
Gross unrealized losses	55	59
Estimated fair value	13,886	24,163
Total available-for-sale securities
Available-for-sale securities:
Amortized cost	1,446,197	1,363,228
Gross unrealized gains	6,516	17,425
Gross unrealized losses	19,491	15,825
Estimated fair value	1,433,222	1,364,828
Held-to-maturity securities - states & political subdivisions
Fair value of securities lent	44,400	44,400
Held-to-maturity securities
Held-to-maturity securities - states & political subdivisions
Amortized cost	4,833	4,833
Gross unrealized gains	15	30
Gross unrealized losses	0	0
Estimated fair value	$ 4,848	$ 4,863