Investments - Amortized Cost and Estimated Fair Value of Available-For-Sale Securities by Contractual Maturity (Details) $ in Thousands	Jun. 30, 2019 USD ($)
Amortized cost
Due in one year or less	$ 62,962
Due after one year through five years	358,056
Due after five years through ten years	127,215
Due after ten years	139,547
Amortized cost	687,780
Estimated fair value
Due in one year or less	63,031
Due after one year through five years	358,508
Due after five years through ten years	127,364
Due after ten years	140,205
Estimated fair value	$ 689,108