Financial Instruments - Marketable Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Due in one year or less	$ 128,751	$ 137,576	[1]
Due after one year	121,754	97,447	[1]
Total	$ 250,505	$ 235,023

[1] See Note 1 for a summary of adjustments.