Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Available-for-sale securities
Amortized cost	$ 992,553
Estimated fair value	961,241
Corporate debt securities
Available-for-sale securities
Amortized cost	600,639	$ 588,536
Gross unrealized gains	4,594	657
Gross unrealized losses	16,545	35,811
Estimated fair value	588,688	553,382
Collateralized debt obligations
Available-for-sale securities
Amortized cost	114,400	107,730
Gross unrealized gains	156	11
Gross unrealized losses	2,088	5,204
Estimated fair value	112,468	102,537
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	106,019	73,855
Gross unrealized gains	1,410	157
Gross unrealized losses	4,709	7,958
Estimated fair value	102,720	66,054
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	153,633	166,412
Gross unrealized gains	69	72
Gross unrealized losses	13,647	16,069
Estimated fair value	140,055	150,415
Other debt securities
Available-for-sale securities
Amortized cost	17,862	24,602
Gross unrealized gains	136	0
Gross unrealized losses	688	2,329
Estimated fair value	17,310	22,273
Available-for-sale securities
Available-for-sale securities
Amortized cost	992,553	961,135
Gross unrealized gains	6,365	897
Gross unrealized losses	37,677	67,371
Estimated fair value	$ 961,241	$ 894,661