Investments - Amortized Cost and Estimated Fair Value of Available-For-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized cost
Due in one year or less	$ 17,393
Due after one year through five years	419,019
Due after five years through ten years	185,849
Due after ten years	276,529
Amortized cost	898,790	$ 724,950
Estimated fair value
Due in one year or less	17,553
Due after one year through five years	435,504
Due after five years through ten years	189,711
Due after ten years	285,468
Estimated fair value	$ 928,236	$ 730,701