Summary of Significant Accounting Policies - Impact on Unaudited Condensed Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 131,702	$ 123,933	$ 121,798	$ 120,942	$ 133,478	$ 123,738	$ 117,026	$ 111,019	$ 498,375	$ 485,261	$ 486,982
Cost of revenues	81,826	79,183	79,595	79,227	90,030	81,139	77,635	74,944	319,831	323,748	335,702
Income from operations	11,401	9,261	8,491	6,791	4,939	10,957	8,225	4,859	35,944	28,980	32,205
Net income	$ 7,477	$ 6,305	$ 5,849	$ 4,928	$ 6,436	$ 7,027	$ 2,409	$ 2,709	24,559	18,581	20,459
Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									494,001	446,619	437,028
Cost of revenues									319,831	290,453	292,121
Software and hardware
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									4,374	38,642	49,954
Cost of revenues									0	$ 33,295	$ 43,581
Accounting Standards Update 2014-09 | ASC Topic 606 Impact
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									25,037
Cost of revenues									25,954
Income from operations									(917)
Net income									(696)
Accounting Standards Update 2014-09 | ASC Topic 606 Impact | Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									0
Cost of revenues									0
Accounting Standards Update 2014-09 | ASC Topic 606 Impact | Software and hardware
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									25,037
Cost of revenues									25,954
Accounting Standards Update 2014-09 | Without ASC Topic 606 Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									523,412
Cost of revenues									345,785
Income from operations									35,027
Net income									23,863
Accounting Standards Update 2014-09 | Without ASC Topic 606 Adoption | Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									494,001
Cost of revenues									319,831
Accounting Standards Update 2014-09 | Without ASC Topic 606 Adoption | Software and hardware
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									29,411
Cost of revenues									$ 25,954