Summary of Significant Accounting Policies - Cummulative Effect Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	$ 38,689	$ 36,178	$ 31,625
Prepaid expenses and other current assets	9,795	9,504	7,609
Deferred cost of revenues	31,568	31,271	31,410
Other assets	6,360	6,290	5,911
Noncontrolling interests subject to put provisions	(171)	(114)	(165)
Accumulated deficit	(416,982)	(415,523)	(421,593)
Noncontrolling interests not subject to put provisions	273	555	(12)	$ 297	$ 625
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	35,288		31,625
Prepaid expenses and other current assets	7,801		7,609
Deferred cost of revenues	31,645		31,410
Other assets	5,981
Noncontrolling interests subject to put provisions	(171)		(165)
Accumulated deficit	(422,699)		(421,593)
Noncontrolling interests not subject to put provisions	295		$ (12)
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	3,401	4,553
Prepaid expenses and other current assets	1,994	1,895
Deferred cost of revenues	(77)	(139)
Other assets	379	379
Noncontrolling interests subject to put provisions	0	51
Accumulated deficit	5,717	6,070
Noncontrolling interests not subject to put provisions	$ (22)	$ 567