Revenue - Adoption of ASC Topic 606 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2016
Condensed Consolidated Balance Sheets
Other assets	$ 10,112	$ 2,163
Assets	320,969	162,754
Accumulated deficit	(192,009)	(146,186)
Total stockholders’ equity (deficit)	257,652	(131,927)	$ (107,006)		$ (78,249)
Total liabilities and stockholders’ equity (deficit)	320,969	162,754
Condensed Consolidated Statements of Operations
Sales and marketing	71,097	50,134	40,818
Total operating expenses	158,688	108,202	87,258
Loss from operations	(46,548)	(29,138)	(31,030)
Loss before income taxes	(51,058)	(30,750)	(32,242)
Net loss	(51,550)	(30,935)	(32,450)
Comprehensive loss	(51,318)	$ (30,947)	$ (32,425)
As would have been reported under previous revenue standards
Condensed Consolidated Balance Sheets
Other assets	4,524
Assets	315,381
Accumulated deficit	(197,597)
Total stockholders’ equity (deficit)	252,064
Total liabilities and stockholders’ equity (deficit)	315,381
Condensed Consolidated Statements of Operations
Sales and marketing	70,958
Total operating expenses	158,549
Loss from operations	(46,409)
Loss before income taxes	(50,919)
Net loss	(51,411)
Comprehensive loss	(51,179)
ASC Topic 606 | Impact of adopting ASC 606 and ASC 340-40
Condensed Consolidated Balance Sheets
Other assets	(5,588)
Assets	(5,588)
Accumulated deficit	(5,588)			$ 5,700
Total stockholders’ equity (deficit)	(5,588)
Total liabilities and stockholders’ equity (deficit)	(5,588)
Condensed Consolidated Statements of Operations
Sales and marketing	(139)
Total operating expenses	(139)
Loss from operations	139
Loss before income taxes	139
Net loss	139
Comprehensive loss	$ 139