Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 300,950	$ 288,386	$ 279,530	$ 267,390	$ 270,418	$ 279,993	$ 269,220	$ 271,073	$ 1,136,256	$ 1,090,704	$ 1,029,353
Gross profit	71,311	72,519	70,227	64,277	68,727	67,781	65,094	61,946	278,335	263,548	240,194
Net income (loss)	$ 1,499	$ 7,528	$ 4,493	$ 5,456	$ 5,047	$ 4,341	$ (2,324)	$ (2,693)	$ 18,979	$ 4,370	$ (33,063)
Net income (loss) per share:
Earnings Per Share, Basic	$ 0.03	$ 0.14	$ 0.08	$ 0.10	$ 0.09	$ 0.08	$ (0.04)	$ (0.05)	$ 0.35	$ 0.08	$ (0.63)
Diluted (in USD per share)	$ 0.03	$ 0.14	$ 0.08	$ 0.10	$ 0.09	$ 0.08	$ (0.04)	$ (0.05)	$ 0.35	$ 0.08	$ (0.63)
Effect of Fourth Quarter Events [Line Items]
Goodwill impairment charge									$ 0
Decrease in contingent consideration liability									$ (677)	$ (10,417)	$ 270

Amount of increase (decrease) in the value of a contingent consideration liability, including, but not limited to, differences arising upon settlement.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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