Investments - Amortized Cost and Fair Value of Available-for-sale Fixed Income Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Amortized Cost
Due in one year or less	$ 176,148
Due after one year through five years	909,286
Due after five years through 10 years	653,649
Due after 10 years	547,547
ABS/CMBS/MBS	760,155
Total available-for-sale	3,046,785	$ 2,945,273
Fair Value
Due in one year or less	174,377
Due after one year through five years	858,578
Due after five years through 10 years	596,523
Due after 10 years	422,989
ABS/CMBS/MBS	665,160
Total available-for-sale	$ 2,717,627	$ 2,666,950