Investments - Amortized Cost and Estimated Fair Value of Available-For-Sale Securities by Contractual Maturity (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Amortized cost
Due in one year or less	$ 32,875
Due after one year through five years	307,299
Due after five years through ten years	128,109
Due after ten years	208,053
Amortized cost	676,336
Estimated fair value
Due in one year or less	32,842
Due after one year through five years	308,682
Due after five years through ten years	129,071
Due after ten years	210,038
Estimated fair value	$ 680,633