Acquisitions (Narrative) (Details) - USD ($) $ in Thousands					1 Months Ended	9 Months Ended		12 Months Ended
	Nov. 08, 2014	Nov. 07, 2014	Jun. 02, 2014	Nov. 09, 2012	Mar. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2014
Business Acquisition [Line Items]
Change in fair value contingent consideration						$ 3,792	$ 0
Goodwill						80,387			$ 80,848
Engage Pty Ltd. [Member]
Business Acquisition [Line Items]
Business acquisition, acquisition date				Nov. 09, 2012
Acquisition costs incurred to acquire entity								$ 500
Assets acquired liabilities assumed, net				$ 800
Potential earn-out consideration				1,700
Change in fair value contingent consideration						700
Engage [Member]
Business Acquisition [Line Items]
Total consideration transferred				$ 10,600
NexGraph, LLC [Member]
Business Acquisition [Line Items]
Business acquisition, acquisition date					Mar. 11, 2014
Acquired Intangible Assets Amount							500
Intangible assets							100
Goodwill							$ 400
Synchronite, LLC [Member]
Business Acquisition [Line Items]
Business acquisition, acquisition date			Jun. 02, 2014
Total consideration transferred			$ 4,100
Acquisition costs incurred to acquire entity						400
Assets acquired liabilities assumed, net			45
Potential earn-out consideration			1,800
Change in fair value contingent consideration						100
Intangible assets			$ 1,451			1,500
Goodwill						2,700
Contact At Once! [Member]
Business Acquisition [Line Items]
Business acquisition, acquisition date		Nov. 07, 2014
Total consideration transferred	$ 67,000
Acquired intangible asset, aggregate cash consideration		$ 66,998
Potential earn-out consideration		4,200
Change in fair value contingent consideration						3,200
Intangible assets		20,400
Goodwill		45,064
Cash payment		42,800
Consideration transferred, shares		$ 20,000
General and administrative expense [Member] | Contact At Once! [Member]
Business Acquisition [Line Items]
Change in fair value contingent consideration						1,900
Cost of revenue [Member] | Contact At Once! [Member]
Business Acquisition [Line Items]
Change in fair value contingent consideration						1,300
Accrued Expenses [Member] | Contact At Once! [Member]
Business Acquisition [Line Items]
Potential earn-out consideration						$ 700