Acquisitions - Additional Information (Detail) - USD ($) $ in Thousands				1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 14, 2016	Nov. 02, 2015	Jun. 01, 2015	Oct. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Purchase price of acquisition							$ 9,745	$ 17,296
Purchase price allocation, goodwill recorded					$ 1,920,742		1,920,742		$ 1,915,690	$ 1,884,920
Contingent consideration arrangement							854	0
Total acquisition costs expensed					900	$ 400	3,400	2,000
Revenue for 12 month period subsequent to acquisition					$ 6,500	$ 1,600	$ 21,000	$ 2,000
Synrevoice [Member]
Business Acquisition [Line Items]
Purchase price of acquisition	$ 9,300
Date of acquisition							Mar. 14, 2016
Purchase price allocation, goodwill recorded	4,893
Preliminary purchase price allocation, intangible assets recorded	$ 6,455
ClientTell [Member]
Business Acquisition [Line Items]
Purchase price of acquisition		$ 38,400
Date of acquisition							Nov. 02, 2015
Purchase price allocation, goodwill recorded		15,004
Preliminary purchase price allocation, intangible assets recorded		26,300
Additional cash to be paid		$ 10,500
Additional cash to be paid over the period in years		5 years
Contingent consideration arrangement		$ 5,400					$ 5,300		$ 5,600
Business acquisition contingent consideration paid							$ 1,200
Magnetic North [Member]
Business Acquisition [Line Items]
Purchase price of acquisition				$ 39,200
Date of acquisition							Oct. 31, 2015
Purchase price allocation, goodwill recorded				24,836
Preliminary purchase price allocation, intangible assets recorded				$ 16,361
Sharp School [Member]
Business Acquisition [Line Items]
Purchase price of acquisition			$ 17,200
Date of acquisition							Jun. 01, 2015
Purchase price allocation, goodwill recorded			8,254
Preliminary purchase price allocation, intangible assets recorded			$ 9,092