Investments - Amortized Cost and Estimated Fair Value of Available-For-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 30,255
Due after one year through five years	420,007
Due after five years through ten years	183,979
Due after ten years	286,158
Amortized cost	920,399	$ 898,790
Estimated fair value
Due in one year or less	30,627
Due after one year through five years	430,590
Due after five years through ten years	186,450
Due after ten years	289,837
Estimated fair value	$ 937,504	$ 928,236