Recent Accounting Standards - Cumulative Effect of ASC Topic 606 Adopted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 123,566	$ 124,097	$ 382,289	$ 377,705
Assets:
Prepaid expenses and other current assets	18,452		18,452		$ 16,271	$ 14,212
Other assets	10,170		10,170		6,701	6,569
Total assets	401,497		401,497			365,007
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	71,422		71,422			78,280
Deferred revenue	23,499		23,499		24,943	22,356
Retained earnings	115,654		115,654		$ 106,203	106,599
Total Liabilities and Stockholders' Equity	401,497		401,497			365,007
Cost of revenue	104,367	105,451	322,838	317,236
Gross profit	19,199	18,646	59,451	60,469
Income tax expense	1,102	643	4,164	5,232
Net income	$ 3,244	$ 3,281	$ 9,451	$ 13,230
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.20	$ 0.20	$ 0.57	$ 0.79
Diluted earnings per share (in dollars per share)	$ 0.20	$ 0.19	$ 0.57	$ 0.78
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 123,727		$ 381,135
Assets:
Prepaid expenses and other current assets	16,816		16,816			14,212
Other assets	10,210		10,210			6,569
Total assets	399,901		399,901
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	70,933		70,933
Deferred revenue	22,489		22,489			22,356
Retained earnings	115,557		115,557			106,599
Total Liabilities and Stockholders' Equity	399,901		399,901
Cost of revenue	104,732		322,349
Gross profit	18,995		58,786
Income tax expense	1,051		3,992
Net income	$ 3,091		$ 8,958
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.19		$ 0.54
Diluted earnings per share (in dollars per share)	$ 0.19		$ 0.54
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ (161)		$ 1,154
Assets:
Prepaid expenses and other current assets	1,636		1,636			2,059
Other assets	(40)		(40)			132
Total assets	1,596		1,596
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	489		489
Deferred revenue	1,010		1,010			2,587
Retained earnings	97		97			$ (396)
Total Liabilities and Stockholders' Equity	1,596		1,596
Cost of revenue	(365)		489
Gross profit	204		665
Income tax expense	51		172
Net income	$ 153		$ 493
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.01		$ 0.03
Diluted earnings per share (in dollars per share)	$ 0.01		$ 0.03