Investments - Cost and Fair Value, Net of Credit Loss Allowance, of Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Available-for-sale securities
Amortized cost	$ 976,358
Estimated fair  value	920,952
Corporate debt securities
Available-for-sale securities
Amortized cost	598,164	$ 588,536
Gross unrealized gains	1,089	657
Gross unrealized losses	30,142	35,811
Estimated fair  value	569,111	553,382
Collateralized debt obligations
Available-for-sale securities
Amortized cost	110,724	107,730
Gross unrealized gains	25	11
Gross unrealized losses	3,828	5,204
Estimated fair  value	106,921	102,537
Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost	77,168	73,855
Gross unrealized gains	87	157
Gross unrealized losses	6,285	7,958
Estimated fair  value	70,970	66,054
Residential mortgage-backed securities
Available-for-sale securities
Amortized cost	173,842	166,412
Gross unrealized gains	39	72
Gross unrealized losses	15,506	16,069
Estimated fair  value	158,375	150,415
Other debt securities
Available-for-sale securities
Amortized cost	16,460	24,602
Gross unrealized gains	0	0
Gross unrealized losses	885	2,329
Estimated fair  value	15,575	22,273
Available-for-sale securities
Available-for-sale securities
Amortized cost	976,358	961,135
Gross unrealized gains	1,240	897
Gross unrealized losses	56,646	67,371
Estimated fair  value	$ 920,952	$ 894,661