Acquisitions - Narrative (Details) - USD ($) $ in Thousands				3 Months Ended
	Feb. 28, 2019	Sep. 30, 2018	Aug. 31, 2018	Mar. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill				$ 834,572	$ 753,718
MicroPact and MyCivic
Business Acquisition [Line Items]
Financial advisory and legal fees				695
MicroPact
Business Acquisition [Line Items]
Cash	$ 1,983
Business acquisition consideration	204,200
Payments to acquire business	197,500
Accrued contingent consideration	7,000
Accrued acquisition holdbacks	1,700
Tangible assets acquired	33,200
Liabilities assumed	21,800
Goodwill	82,029
Identifiable intangible assets	$ 118,843
Weighted average useful life (in years)	10 years
Deferred tax liabilities, net	$ 6,144
Revenue, actual				5,500
Income (loss), actual				$ (400)
MyCivic
Business Acquisition [Line Items]
Business acquisition consideration			$ 3,700
Payments to acquire business			$ 3,600
Accrued working capital hold back		$ 90