Fair Value of Financial Instruments - Contractual Maturity (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	$ 214,516
After One Year through Two Years	28,037
Over Two Years	10,015
Fair Value	252,568
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	20,933
After One Year through Two Years	0
Over Two Years	0
Fair Value	20,933
US government agencies
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	159,019
After One Year through Two Years	0
Over Two Years	0
Fair Value	159,019
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	32,783
After One Year through Two Years	27,037
Over Two Years	9,449
Fair Value	69,269
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Mature within One Year	1,781
After One Year through Two Years	1,000
Over Two Years	566
Fair Value	$ 3,347