Business Combinations, Goodwill and Intangible Assets Business Combinations, Goodwill and Intangible Assets - Purchase Price Allocation - SCIO (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jul. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill	$ 357,533		$ 204,481	$ 186,770
SCIO
Business Acquisition [Line Items]
Cash and cash equivalents		$ 10,026
Restricted cash		2,790
Accounts receivable		19,924
Other current assets		1,798
Property and equipment		1,824
Other assets		1,751
Assets		118,513
Current liabilities		(11,679)
Deferred tax liabilities		(19,052)
Other non-current liabilities		(200)
Liabilities		30,931
Net assets acquired		87,582
Goodwill		157,909
Total purchase consideration		245,491
Customer relationships | SCIO
Business Acquisition [Line Items]
Identifiable Intangible Assets		47,800
Developed technology | SCIO
Business Acquisition [Line Items]
Identifiable Intangible Assets		28,900
Trade names and trademarks | SCIO
Business Acquisition [Line Items]
Identifiable Intangible Assets		$ 3,700