Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Available-for-sale securities:
Amortized cost	$ 1,127,741
Estimated fair value	1,112,707
Held-to-maturity securities:
Amortized cost	1,132,574	$ 1,070,789
Gross unrealized gains	9,133	6,907
Gross unrealized losses	24,098	29,147
Estimated fair value	1,117,609	1,048,549
Fair value of securities lent	12,300
Corporate debt securities
Available-for-sale securities:
Amortized cost	696,519	647,861
Gross unrealized gains	6,111	4,767
Gross unrealized losses	7,298	8,685
Estimated fair value	695,332	643,943
Held-to-maturity securities:
Fair value of securities lent	12,200	7,300
Collateralized debt obligations
Available-for-sale securities:
Amortized cost	107,667	114,142
Gross unrealized gains	191	372
Gross unrealized losses	393	387
Estimated fair value	107,465	114,127
Commercial mortgage-backed securities
Available-for-sale securities:
Amortized cost	120,342	126,509
Gross unrealized gains	1,930	1,458
Gross unrealized losses	2,246	2,985
Estimated fair value	120,026	124,982
Residential mortgage-backed securities
Available-for-sale securities:
Amortized cost	157,872	150,212
Gross unrealized gains	310	62
Gross unrealized losses	13,703	16,462
Estimated fair value	144,479	133,812
Other debt securities
Available-for-sale securities:
Amortized cost	36,362	27,232
Gross unrealized gains	366	147
Gross unrealized losses	458	628
Estimated fair value	36,270	26,751
U.S. Treasury
Available-for-sale securities:
Amortized cost	8,979
Gross unrealized gains	156
Gross unrealized losses	0
Estimated fair value	9,135
Total available-for-sale securities
Available-for-sale securities:
Amortized cost	1,127,741	1,065,956
Gross unrealized gains	9,064	6,806
Gross unrealized losses	24,098	29,147
Estimated fair value	1,112,707	1,043,615
Held-to-maturity securities
Held-to-maturity securities:
Amortized cost	4,833	4,833
Gross unrealized gains	69	101
Gross unrealized losses	0	0
Estimated fair value	$ 4,902	$ 4,934