Recent Accounting Standards - Cumulative Effect of ASC Topic 606 Adopted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Assets:
Prepaid expenses and other current assets	$ 14,664		$ 14,664		$ 16,271	$ 14,212
Other assets	8,250		8,250		6,701	6,569
Total assets	389,095		389,095			365,007
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	72,893		72,893			78,280
Deferred revenue	18,694		18,694		24,943	22,356
Retained earnings	112,410		112,410		$ 106,203	106,599
Total Liabilities and Stockholders' Equity	389,095		389,095			365,007
Revenue	133,691	$ 131,161	258,723	$ 253,608
Cost of revenue	111,118	108,726	218,471	211,785
Gross profit	22,573	22,435	40,252	41,823
Income tax expense	1,332	2,597	3,062	4,589
Net income	$ 3,575	$ 5,863	$ 6,207	$ 9,949
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.22	$ 0.35	$ 0.37	$ 0.59
Diluted earnings per share (in dollars per share)	$ 0.22	$ 0.35	$ 0.37	$ 0.59
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets:
Prepaid expenses and other current assets	$ 13,461		$ 13,461			14,212
Other assets	8,239		8,239			6,569
Total assets	387,881		387,881
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	72,039		72,039
Deferred revenue	18,278		18,278			22,356
Retained earnings	112,466		112,466			106,599
Total Liabilities and Stockholders' Equity	387,881		387,881
Revenue	132,359		257,408
Cost of revenue	110,063		217,617
Gross profit	22,296		39,791
Income tax expense	1,257		2,941
Net income	$ 3,373		$ 5,867
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.20		$ 0.35
Diluted earnings per share (in dollars per share)	$ 0.20		$ 0.35
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	$ 1,203		$ 1,203			2,059
Other assets	11		11			132
Total assets	1,214		1,214
Liabilities and Stockholders’ Equity:
Accounts payable and accrued expenses	854		854
Deferred revenue	416		416			2,587
Retained earnings	(56)		(56)			$ (396)
Total Liabilities and Stockholders' Equity	1,214		1,214
Revenue	1,332		1,315
Cost of revenue	1,055		854
Gross profit	277		461
Income tax expense	75		121
Net income	$ 202		$ 340
Per common share data:
Basic earnings per share (in dollars per share)	$ 0.02		$ 0.02
Diluted earnings per share (in dollars per share)	$ 0.02		$ 0.02