Business Combination - NDNQI (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 10, 2014	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	402,934	$ 385,750	$ 384,341
NDNQI
Business Acquisitions
Acquisitions of businesses, net of cash acquired	$ 24,900
Preliminary allocation of fair value of the acquisition:
Current assets			478
Goodwill			14,647
Intangibles:
Total assets acquired			31,211
Liabilities assumed:
Deferred revenue			6,180
Other current liabilities			181
Net assets acquired			24,850
NDNQI | Book value
Liabilities assumed:
Deferred revenue	6,500
NDNQI | Fair value
Liabilities assumed:
Deferred revenue	6,200
NDNQI | Fair Value Adjustment
Liabilities assumed:
Deferred revenue	$ 300
NDNQI | Trade names
Intangibles:
Intangibles			300
NDNQI | Customer relationships
Intangibles:
Intangibles			2,800
NDNQI | Proprietary technology
Intangibles:
Intangibles			11,800
NDNQI | Software
Preliminary allocation of fair value of the acquisition:
Property and equipment			1,186
General and administrative | NDNQI
Other information
Transaction expenses			$ 246