Business Combinations, Clarity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Combinations [Abstract]
Adjustment to fair value of contingent consideration for purchase of business	$ 3,235	$ (1,679)	$ 445
Allocation of Total Purchase Price Consideration [Abstract]
Goodwill	$ 305,238	$ 275,205	$ 269,383
Clarity Consultants [Member]
Business Combinations [Abstract]
Date of acquisition	Jun. 22, 2017
Cash paid for acquisition	$ 30,700
Common stock issued	7,300
Net working capital settlement	(400)
Initial fair value estimate of additional earnings-based contingent consideration	$ 4,100
Period to realize additional earnings-based contingent consideration	12 months
Adjustment to fair value of contingent consideration for purchase of business	$ 4,000
Current contingent consideration liability	8,100
Maximum potential additional earnings-based contingent consideration	9,200
Transaction costs	900
Common stock value attributed to future compensation	900
Allocation of Total Purchase Price Consideration [Abstract]
Acquired tangible assets	6,000
Acquired intangible assets	15,000
Liabilities assumed	(3,600)
Goodwill	24,300
Total purchase price	41,700
Tax deductible amount of Goodwill	$ 22,100