Fair Value of Financial Instruments - Contractual Maturity (Details) $ in Thousands	Jun. 30, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	$ 149,902
After One Year through Two Years	31,275
Over Two Years	12,117
Fair Value	193,294
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	3,245
After One Year through Two Years	0
Over Two Years	0
Fair Value	3,245
US government agencies
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	118,057
After One Year through Two Years	15,256
Over Two Years	0
Fair Value	133,313
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	25,978
After One Year through Two Years	15,169
Over Two Years	12,117
Fair Value	53,264
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	2,622
After One Year through Two Years	850
Over Two Years	0
Fair Value	$ 3,472