Investments - Amortized Cost and Estimated Fair Value of Available-For-Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized cost
Due in one year or less	$ 16,625
Due after one year through five years	421,401
Due after five years through ten years	187,492
Due after ten years	274,424
Amortized cost	899,942	$ 898,790
Estimated fair value
Due in one year or less	16,813
Due after one year through five years	433,929
Due after five years through ten years	189,327
Due after ten years	278,253
Estimated fair value	$ 918,322	$ 928,236