Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Available-for-sale securities:
Amortized cost	$ 1,061,705
Estimated fair value	1,056,992
Held-to-maturity securities:
Amortized cost	1,066,538
Gross unrealized gains	16,397
Gross unrealized losses	21,110
Estimated fair value	1,061,825
Fair value of loaned securities	8,135	$ 0
Corporate debt securities
Available-for-sale securities:
Amortized cost	643,288	600,639
Gross unrealized gains	11,730	4,594
Gross unrealized losses	6,505	16,545
Estimated fair value	648,513	588,688
Collateralized debt obligations
Available-for-sale securities:
Amortized cost	110,641	114,400
Gross unrealized gains	247	156
Gross unrealized losses	616	2,088
Estimated fair value	110,272	112,468
Commercial mortgage-backed securities
Available-for-sale securities:
Amortized cost	136,370	106,019
Gross unrealized gains	3,555	1,410
Gross unrealized losses	2,570	4,709
Estimated fair value	137,355	102,720
Residential mortgage- backed securities
Available-for-sale securities:
Amortized cost	145,864	153,633
Gross unrealized gains	292	69
Gross unrealized losses	10,981	13,647
Estimated fair value	135,175	140,055
Other debt securities
Available-for-sale securities:
Amortized cost	25,542	17,862
Gross unrealized gains	573	136
Gross unrealized losses	438	688
Estimated fair value	25,677	17,310
Total available-for-sale securities
Available-for-sale securities:
Amortized cost	1,061,705	992,553
Gross unrealized gains	16,397	6,365
Gross unrealized losses	21,110	37,677
Estimated fair value	1,056,992	$ 961,241
Held-to-maturity securities
Held-to-maturity securities:
Amortized cost	4,833
Gross unrealized gains	0
Gross unrealized losses	0
Estimated fair value	$ 4,833