Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Available-for-sale securities:
Amortized cost	$ 1,001,906
Estimated fair value	969,645
Held-to-maturity securities:
Amortized cost	1,006,739
Gross unrealized gains	5,613
Gross unrealized losses	37,874
Estimated fair value	974,478
Corporate debt securities
Available-for-sale securities:
Amortized cost	606,788	$ 600,639
Gross unrealized gains	3,872	4,594
Gross unrealized losses	15,365	16,545
Estimated fair value	595,295	588,688
Collateralized debt obligations
Available-for-sale securities:
Amortized cost	114,223	114,400
Gross unrealized gains	244	156
Gross unrealized losses	1,379	2,088
Estimated fair value	113,088	112,468
Commercial mortgage-backed securities
Available-for-sale securities:
Amortized cost	113,857	106,019
Gross unrealized gains	1,329	1,410
Gross unrealized losses	4,306	4,709
Estimated fair value	110,880	102,720
Residential mortgage- backed securities
Available-for-sale securities:
Amortized cost	148,917	153,633
Gross unrealized gains	53	69
Gross unrealized losses	16,183	13,647
Estimated fair value	132,787	140,055
Other debt securities
Available-for-sale securities:
Amortized cost	17,077	17,862
Gross unrealized gains	115	136
Gross unrealized losses	636	688
Estimated fair value	16,556	17,310
U.S. Treasury
Available-for-sale securities:
Amortized cost	1,044
Gross unrealized gains	0
Gross unrealized losses	5
Estimated fair value	1,039
Available-for-sale securities
Available-for-sale securities:
Amortized cost	1,001,906	992,553
Gross unrealized gains	5,613	6,365
Gross unrealized losses	37,874	37,677
Estimated fair value	969,645	$ 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

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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