Business Acquisition (Narrative) (Details)			3 Months Ended
	Aug. 06, 2018 USD ($)	Nov. 30, 2016 USD ($)	Sep. 30, 2018 USD ($)	Apr. 02, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 101,965,000			$ 85,463,000
Meural
Business Acquisition [Line Items]
Total purchase price	$ 22,200,000
Purchase price, cash paid			14,400,000
Prior equity interest settlement	1,500,000
Fair value of contingent consideration	6,300,000		$ 5,900,000
Contingent consideration liability, maximum amount of each milestone	3,500,000
Goodwill	16,502,000
Non-current deferred income taxes	815,000
Intangibles	4,700,000
Meural | Developed Technology Rights
Business Acquisition [Line Items]
Intangibles	3,000,000
Meural | Trade Names
Business Acquisition [Line Items]
Intangibles	$ 600,000
Acquired intangible assets, estimated useful life	3 years
Meural | Customer Relationships
Business Acquisition [Line Items]
Intangibles	$ 600,000
Acquired intangible assets, estimated useful life	2 years
Meural | Database
Business Acquisition [Line Items]
Intangibles	$ 500,000
Acquired intangible assets, estimated useful life	7 years
Meural | Technology
Business Acquisition [Line Items]
Acquired intangible assets, estimated useful life	7 years
Placemeter
Business Acquisition [Line Items]
Total purchase price		$ 9,600,000
Purchase price, cash paid				$ 800,000	$ 8,800,000
Goodwill		3,742,000
Intangibles		6,000,000
Deferred tax liabilities		308,000
Placemeter | Database
Business Acquisition [Line Items]
Intangibles		200,000
Placemeter | Technology
Business Acquisition [Line Items]
Intangibles		$ 5,500,000
Acquired intangible assets, estimated useful life		4 years
Placemeter | Non-compete Agreements
Business Acquisition [Line Items]
Intangibles		$ 300,000
Discount rate used to calculate present value of future cash flows		20.00%
Acquired intangible assets, estimated useful life		3 years
Domestic Tax Authority | Meural
Business Acquisition [Line Items]
Goodwill deductible for income tax purposes	$ 0
Domestic Tax Authority | Placemeter
Business Acquisition [Line Items]
Goodwill deductible for income tax purposes		$ 0
State and Local Jurisdiction | Meural
Business Acquisition [Line Items]
Goodwill deductible for income tax purposes	$ 0
State and Local Jurisdiction | Placemeter
Business Acquisition [Line Items]
Goodwill deductible for income tax purposes		$ 0
Measurement Input, Discount Rate | Placemeter
Business Acquisition [Line Items]
Intangible asset, measurement input		0.150
Measurement Input, Discount Rate | Minimum | Meural
Business Acquisition [Line Items]
Intangible asset, measurement input	0.160
Measurement Input, Discount Rate | Maximum | Meural
Business Acquisition [Line Items]
Intangible asset, measurement input	0.190