Financial Instruments - Amortized Cost and Estimated Fair Value of Marketable Securities, by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 864,958	$ 864,003
Due after one year through five years	159,148	173,369
Amortized Cost	1,024,106	1,037,372
Estimated Fair Value
Due in one year or less	867,124	863,063
Due after one year through five years	159,987	172,990
Total	$ 1,027,111	$ 1,036,053