Investments - Amortized Cost and Estimated Fair Value of Available-For-Sale Securities by Contractual Maturity (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Amortized cost
Due in one year or less	$ 13,157
Due after one year through five years	419,336
Due after five years through ten years	160,245
Due after ten years	295,198
Amortized cost	887,936
Estimated fair value
Due in one year or less	13,229
Due after one year through five years	431,758
Due after five years through ten years	161,148
Due after ten years	301,919
Estimated fair value	$ 908,054