Fair Value of Financial Instruments - Contractual Maturity (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	$ 255,796
After One Year through Two Years	52,163
Over Two Years	7,038
Fair Value	314,997
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	26,140
After One Year through Two Years	0
Over Two Years	0
Fair Value	26,140
US government agencies
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	184,336
After One Year through Two Years	16,859
Over Two Years	2,161
Fair Value	203,356
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	39,158
After One Year through Two Years	26,833
Over Two Years	4,877
Fair Value	70,868
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	6,162
After One Year through Two Years	8,471
Over Two Years	0
Fair Value	$ 14,633