Acquisitions (Narrative) (Details) - USD ($)								3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 28, 2018	Mar. 21, 2018	Nov. 16, 2017	Jul. 12, 2017	Apr. 21, 2017	Jan. 10, 2017	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Business Acquisition [Line Items]
Adjustment to intangibles											$ (15,000,000)
Payments to acquire business, net of cash acquired											45,362,000	$ 37,096,000
Goodwill	$ 157,389,000							$ 157,389,000	$ 157,389,000		157,389,000		$ 154,607,000
Measurement period expenses								100,000		$ 0	200,000	0
Transaction costs								3,140,000		2,278,000	6,242,000	5,969,000
Interfax Communications Limited [Member]
Business Acquisition [Line Items]
Adjustment to intangibles								(4,200,000)
Adjustment to deferred tax liabilities								3,100,000
Adjustment to other current assets								(100,000)
Adjustment to accounts payable and accrued expenses and other								100,000
Cash consideration transferred			$ 35,000,000
Contingent consideration, fair value			$ 5,000,000
Cash holdback payable, payment period			18 months
Recorded revenue									4,000,000
Goodwill			$ 16,141,000
Goodwill expected to be tax deductible			2,300,000
RO Innovation, Inc. [Member]
Business Acquisition [Line Items]
Cash holdback payable, payment period		1 year
Recorded revenue	0
Payments to acquire business, net of cash acquired		$ 12,500,000
Cash holdback payable		1,800,000
Goodwill		3,170,000
Goodwill expected to be tax deductible		$ 2,700,000
Omtool, Ltd [Member]
Business Acquisition [Line Items]
Payments to acquire business, net of cash acquired							$ 19,300,000
Cash acquired							3,000,000
Goodwill							$ 14,081,000
RightAnswers, Inc. [Member]
Business Acquisition [Line Items]
Cash holdback payable, payment period						1 year
Payments to acquire business, net of cash acquired						$ 17,400,000
Cash holdback payable						2,500,000
Cash acquired						100,000
Goodwill						$ 15,680,000
Waterfall International Inc. [Member]
Business Acquisition [Line Items]
Cash holdback payable, payment period					18 months
Payments to acquire business, net of cash acquired					$ 24,400,000
Cash holdback payable					1,500,000
Cash acquired					400,000
Earnout consideration liability					3,000,000
Goodwill					18,575,000
Goodwill expected to be tax deductible					$ 3,700,000
Qvidian Corporation [Member]
Business Acquisition [Line Items]
Adjustment to accrued expense and other								(100,000)
Adjustment to deferred revenue								(100,000)
Payments to acquire business, net of cash acquired				$ 50,000,000
Goodwill				$ 21,225,000
LeadLander, HipCricket, Omtool and RightAnswers [Member]
Business Acquisition [Line Items]
Goodwill	88,900,000							88,900,000	88,900,000		88,900,000
Omtool Ltd., RightAnswers Inc., Qvidian Corporation and Interfax Communications Limited [Member]
Business Acquisition [Line Items]
Goodwill expected to be tax deductible	$ 0							0	$ 0		0
Series of Business Acquisitions [Member]
Business Acquisition [Line Items]
Transaction costs								$ 1,200,000		$ 1,000,000	$ 2,600,000	$ 2,600,000
Marketech [Member] | Interfax Communications Limited [Member]
Business Acquisition [Line Items]
Payments to acquire certain assets and intangible assets			$ 2,000,000