Acquisitions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Sep. 30, 2018	May 15, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill	$ 149,762		$ 125,272	$ 116,239
WriteLab, Inc.
Business Acquisition [Line Items]
Cash		$ 234
Accounts receivable		302
Other acquired assets		151
Total acquired intangible assets		11,550
Total identifiable assets acquired		12,237
Liabilities assumed		(2,133)
Net identifiable assets acquired		10,104
Goodwill		24,780
Total fair value of purchase consideration		34,884
Trade names | WriteLab, Inc.
Business Acquisition [Line Items]
Total acquired intangible assets		140
Domain names | WriteLab, Inc.
Business Acquisition [Line Items]
Total acquired intangible assets		180
Non-compete agreements | WriteLab, Inc.
Business Acquisition [Line Items]
Total acquired intangible assets		220
Developed technologies | WriteLab, Inc.
Business Acquisition [Line Items]
Total acquired intangible assets		5,790
Content library | WriteLab, Inc.
Business Acquisition [Line Items]
Total acquired intangible assets		$ 5,220