Significant Accounting Policies (Details 3) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Property and equipment
Costs incurred during the period	$ 7,529	$ 7,082	$ 6,466	$ 6,134	$ 5,403	$ 5,726	$ 5,818	$ 5,674	$ 27,211	$ 22,621	$ 19,472
Capitalized Content Development Costs
Useful life of capitalized content development costs									5 years
Impairment of Long-Lived Assets
Impairments of long-lived assets									$ 0	0
Segment and Geographic Information
Number of Reportable Segments | segment									1
Minimum
Revenue Recognition and Deferred Revenue
Term of revenue contracts									10 years
Maximum
Revenue Recognition and Deferred Revenue
Term of revenue contracts									15 years
Computer hardware | Minimum
Property and equipment
Useful life									3 years
Computer hardware | Maximum
Property and equipment
Useful life									5 years
Furniture and office equipment | Minimum
Property and equipment
Useful life									5 years
Furniture and office equipment | Maximum
Property and equipment
Useful life									7 years
Leasehold improvements | Minimum
Property and equipment
Useful life									4 years
Leasehold improvements | Maximum
Property and equipment
Useful life									10 years
Internally-developed software
Property and equipment
Useful life									3 years
Related amortization expense									$ 1,600	1,400	$ 1,500
Net book value	$ 4,500				$ 3,300				$ 4,500	$ 3,300