Recent Accounting Standards - Cumulative Effect of ASC Topic 606 Adopted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Assets:
Prepaid expenses and other current assets	$ 19,191		$ 16,271
Other assets	7,569		6,701
Total assets	363,075			$ 365,007
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	74,889			78,280
Deferred revenue	22,563		24,943
Retained earnings	108,835		$ 106,203
Total Liabilities and Stockholders' Equity	363,075			365,007
Revenue	125,032	$ 122,447
Cost of revenue	107,353	103,059
Gross profit	17,679	19,388
Income tax expense	1,730	1,992
Net income	$ 2,632	$ 4,086
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.16	$ 0.24
Diluted earnings per share (in dollars per share)	$ 0.16	$ 0.24
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets:
Prepaid expenses and other current assets	$ 16,934			14,212
Other assets	7,483			6,569
Total assets	360,732
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	75,090
Deferred revenue	19,761			22,356
Retained earnings	109,093			106,599
Total Liabilities and Stockholders' Equity	360,732
Revenue	125,049
Cost of revenue	107,554
Gross profit	17,495
Income tax expense	1,684
Net income	$ 2,494
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.15
Diluted earnings per share (in dollars per share)	$ 0.15
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Assets:
Prepaid expenses and other current assets	$ 2,257			2,059
Other assets	86			132
Total assets	2,343
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	(201)
Deferred revenue	2,802			2,587
Retained earnings	(258)			$ (396)
Total Liabilities and Stockholders' Equity	2,343
Revenue	(17)
Cost of revenue	(201)
Gross profit	184
Income tax expense	46
Net income	$ 138
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.01
Diluted earnings per share (in dollars per share)	$ 0.01