Fair Value of Financial Instruments - Contractual Maturity (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	$ 261,111
Mature after One Year through Two Years	50,481
Mature over Two Years	294
Fair Value	311,886
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	28,749
Mature after One Year through Two Years	0
Mature over Two Years	0
Fair Value	28,749
US government agencies
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	181,132
Mature after One Year through Two Years	18,730
Mature over Two Years	0
Fair Value	199,862
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	37,791
Mature after One Year through Two Years	31,268
Mature over Two Years	294
Fair Value	69,353
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	13,439
Mature after One Year through Two Years	483
Mature over Two Years	0
Fair Value	$ 13,922