Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Available-for-sale securities:
Amortized cost	$ 1,065,216
Estimated fair value	1,065,833
Held-to-maturity securities:
Amortized cost	1,070,049	$ 1,070,789
Gross unrealized gains	17,341	6,907
Gross unrealized losses	16,681	29,147
Estimated fair value	1,070,709	1,048,549
Fair value of securities lent	56,200
Corporate debt securities
Available-for-sale securities:
Amortized cost	663,602	647,861
Gross unrealized gains	12,714	4,767
Gross unrealized losses	3,230	8,685
Estimated fair value	673,086	643,943
Held-to-maturity securities:
Fair value of securities lent		7,300
Collateralized debt obligations
Available-for-sale securities:
Amortized cost	90,481	114,142
Gross unrealized gains	238	372
Gross unrealized losses	168	387
Estimated fair value	90,551	114,127
Commercial mortgage-backed securities
Available-for-sale securities:
Amortized cost	121,053	126,509
Gross unrealized gains	2,868	1,458
Gross unrealized losses	1,710	2,985
Estimated fair value	122,211	124,982
Residential mortgage-backed securities
Available-for-sale securities:
Amortized cost	151,730	150,212
Gross unrealized gains	736	62
Gross unrealized losses	11,300	16,462
Estimated fair value	141,166	133,812
Other debt securities
Available-for-sale securities:
Amortized cost	31,091	27,232
Gross unrealized gains	622	147
Gross unrealized losses	264	628
Estimated fair value	31,449	26,751
U.S. Treasury
Available-for-sale securities:
Amortized cost	7,259
Gross unrealized gains	120
Gross unrealized losses	9
Estimated fair value	7,370
Total available-for-sale securities
Available-for-sale securities:
Amortized cost	1,065,216	1,065,956
Gross unrealized gains	17,298	6,806
Gross unrealized losses	16,681	29,147
Estimated fair value	1,065,833	1,043,615
Held-to-maturity securities:
Fair value of securities lent	35,900
Held-to-maturity securities
Held-to-maturity securities:
Amortized cost	4,833	4,833
Gross unrealized gains	43	101
Gross unrealized losses	0	0
Estimated fair value	$ 4,876	$ 4,934