Acquisitions - Summary of Estimated Fair Values of Assets Acquired and Liabilities Assumed at Acquisition (Detail) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 09, 2016	Mar. 14, 2016	Dec. 31, 2015	Nov. 02, 2015	Oct. 31, 2015	Jun. 01, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
GOODWILL		$ 1,916,192			$ 1,915,690				$ 1,884,920
911 ETC [Member]
Business Acquisition [Line Items]
Working Capital			$ 743
Property and equipment			135
Other assets, net			0
Intangible assets			6,484
GOODWILL			5,277
Total assets acquired			12,639
Non-current deferred taxes			0
Long-term liabilities			2,464
Total liabilities assumed			2,464
Net assets acquired			$ 10,175
ClientTell [Member]
Business Acquisition [Line Items]
Working Capital						$ 501
Property and equipment						429
Other assets, net						2
Intangible assets						26,300
GOODWILL	[1]					15,004
Total assets acquired						42,236
Non-current deferred taxes						0
Long-term liabilities						3,828
Total liabilities assumed						3,828
Net assets acquired						$ 38,408
Synrevoice [Member]
Business Acquisition [Line Items]
Working Capital				$ (2,118)
Property and equipment				21
Other assets, net				0
Intangible assets				6,455
GOODWILL				4,893
Total assets acquired				9,251
Non-current deferred taxes				0
Long-term liabilities				0
Total liabilities assumed				0
Net assets acquired				$ 9,251
Magnetic North [Member]
Business Acquisition [Line Items]
Working Capital							$ 55
Property and equipment							574
Other assets, net							0
Intangible assets							16,361
GOODWILL							25,094
Total assets acquired							42,084
Non-current deferred taxes							2,927
Long-term liabilities							0
Total liabilities assumed							2,927
Net assets acquired							$ 39,157
Sharp School [Member]
Business Acquisition [Line Items]
Working Capital								$ (982)
Property and equipment								782
Other assets, net								77
Intangible assets								9,092
GOODWILL								8,203
Total assets acquired								17,172
Non-current deferred taxes								0
Long-term liabilities								0
Total liabilities assumed								0
Net assets acquired								$ 17,172

[1] 15.0