Acquisitions - 2017 Acquisitions (Details) - USD ($) $ in Thousands					1 Months Ended	2 Months Ended	6 Months Ended	8 Months Ended	12 Months Ended
	Nov. 16, 2017	Jul. 13, 2017	Apr. 21, 2017	Jan. 11, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Purchase price paid									$ 110,324		$ 12,151	$ 7,664
Omtool, Ltd
Business Acquisition [Line Items]
Purchase price paid				$ 19,300
Revenue since date of acquisition										$ 10,700
RightAnswers
Business Acquisition [Line Items]
Purchase price paid			$ 17,400
Revenue since date of acquisition								$ 5,600
Cash holdback payable			$ 2,500
Cash holdback, payment period (in months)			1 year
Earnout consideration liability			$ 4,000
Earnout payment					$ 1,000
Consideration liability						$ 2,000	$ 2,000	$ 2,000	$ 2,000	$ 2,000
Cash acquired			$ 100
Waterfall
Business Acquisition [Line Items]
Purchase price paid		$ 24,400
Revenue since date of acquisition							$ 5,500
Cash holdback payable		$ 1,500
Cash holdback, payment period (in months)		18 months
Earnout consideration liability		$ 3,000
Cash acquired		$ 400
Qvidian
Business Acquisition [Line Items]
Purchase price paid	$ 50,000
Revenue since date of acquisition						$ 1,200