Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Available-for-sale securities:
Amortized cost	$ 1,133,258
Estimated fair value	1,126,148
Held-to-maturity securities:
Amortized cost	1,138,091	$ 1,070,789
Gross unrealized gains	13,592	6,907
Gross unrealized losses	20,705	29,147
Estimated fair value	1,130,978	1,048,549
Fair value of securities lent	35,500
Corporate debt securities
Available-for-sale securities:
Amortized cost	700,045	647,861
Gross unrealized gains	9,919	4,767
Gross unrealized losses	4,745	8,685
Estimated fair value	705,219	643,943
Held-to-maturity securities:
Fair value of securities lent		7,300
Collateralized debt obligations
Available-for-sale securities:
Amortized cost	108,904	114,142
Gross unrealized gains	275	372
Gross unrealized losses	268	387
Estimated fair value	108,911	114,127
Commercial mortgage-backed securities
Available-for-sale securities:
Amortized cost	123,616	126,509
Gross unrealized gains	2,483	1,458
Gross unrealized losses	1,907	2,985
Estimated fair value	124,192	124,982
Residential mortgage-backed securities
Available-for-sale securities:
Amortized cost	157,688	150,212
Gross unrealized gains	362	62
Gross unrealized losses	13,429	16,462
Estimated fair value	144,621	133,812
Other debt securities
Available-for-sale securities:
Amortized cost	35,756	27,232
Gross unrealized gains	498	147
Gross unrealized losses	327	628
Estimated fair value	35,927	26,751
U.S. Treasury
Available-for-sale securities:
Amortized cost	7,249
Gross unrealized gains	55
Gross unrealized losses	26
Estimated fair value	7,278
Total available-for-sale securities
Available-for-sale securities:
Amortized cost	1,133,258	1,065,956
Gross unrealized gains	13,592	6,806
Gross unrealized losses	20,702	29,147
Estimated fair value	1,126,148	1,043,615
Held-to-maturity securities:
Fair value of securities lent	18,400
Held-to-maturity securities
Held-to-maturity securities:
Amortized cost	4,833	4,833
Gross unrealized gains	0	101
Gross unrealized losses	3	0
Estimated fair value	$ 4,830	$ 4,934