Investments - Amortized Cost and Fair Value of Available-for-sale Fixed Income Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Amortized Cost
Due in one year or less	$ 157,831
Due after one year through five years	904,769
Due after five years through 10 years	674,761
Due after 10 years	548,021
ABS/CMBS/MBS	769,009
Total available-for-sale	3,054,391	$ 2,945,273
Fair Value
Due in one year or less	156,463
Due after one year through five years	874,375
Due after five years through 10 years	654,068
Due after 10 years	464,476
ABS/CMBS/MBS	706,467
Total available-for-sale	$ 2,855,849	$ 2,666,950