Investments - Amortized Cost and Estimated Fair Value of Available-For-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 20,580
Due after one year through five years	429,579
Due after five years through ten years	194,317
Due after ten years	270,558
Amortized cost	915,034	$ 898,790
Estimated fair value
Due in one year or less	20,856
Due after one year through five years	442,409
Due after five years through ten years	197,441
Due after ten years	276,092
Estimated fair value	$ 936,798	$ 928,236