Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Available-for-sale securities
Amortized cost	$ 978,108
Estimated fair  value	915,237
Corporate debt securities
Available-for-sale securities
Amortized cost	596,538	$ 588,536
Gross unrealized gains	776	657
Gross unrealized losses	30,347	35,811
Estimated fair  value	566,967	553,382
Collateralized debt obligations
Available-for-sale securities
Amortized cost	113,252	107,730
Gross unrealized gains	64	11
Gross unrealized losses	2,828	5,204
Estimated fair  value	110,488	102,537
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	78,491	73,855
Gross unrealized gains	90	157
Gross unrealized losses	6,549	7,958
Estimated fair  value	72,032	66,054
Residential mortgage- backed securities
Available-for-sale securities
Amortized cost	173,529	166,412
Gross unrealized gains	11	72
Gross unrealized losses	23,146	16,069
Estimated fair  value	150,394	150,415
Other debt securities
Available-for-sale securities
Amortized cost	16,298	24,602
Gross unrealized gains	0	0
Gross unrealized losses	942	2,329
Estimated fair  value	15,356	22,273
Available-for-sale securities
Available-for-sale securities
Amortized cost	978,108	961,135
Gross unrealized gains	941	897
Gross unrealized losses	63,812	67,371
Estimated fair  value	$ 915,237	$ 894,661