Business Combination - HPI (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Sep. 01, 2015	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisitions
Acquisitions of businesses, net of cash acquired			$ 12,146	$ 28,177	$ 2,813
Preliminary allocation of fair value of the acquisition:
Goodwill		$ 411,203	411,203	$ 402,934	$ 385,750	$ 384,341
HPI
Business Acquisitions
Purchase price	$ 13,000
Acquisitions of businesses, net of cash acquired	$ 11,700	12,100
Working capital adjustments paid		426
Preliminary allocation of fair value of the acquisition:
Current assets		2,213	2,213
Property and equipment		23	23
Goodwill		8,269	8,269
Intangibles:
Total assets acquired		14,195	14,195
Liabilities assumed:
Deferred revenue		1,753	1,753
Other current liabilities		296	296
Net assets acquired		12,146	12,146
HPI | Trade names
Intangibles:
Intangibles		280	280
HPI | Customer relationships
Intangibles:
Intangibles		2,320	2,320
HPI | Other
Intangibles:
Intangibles		$ 1,090	1,090
General and administrative | HPI
Other information
Transaction expenses			$ 346