Summary of Significant Accounting Policies - Liquidity (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended				6 Months Ended		12 Months Ended
	Feb. 29, 2020	Oct. 31, 2019	Oct. 31, 2018	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Cash and cash equivalents				$ 45,430		$ 95,644		$ 45,430	$ 95,644	$ 45,430	$ 43,531	$ 95,615
Long-term debt				53,828				53,828		53,828	53,464
Remaining amount available to borrow				0				0		0
Aggregate minimum liquidity balance								10,000			10,000
Aggregate minimum liquidity balance at quarter end								20,000			20,000
Net losses				(1,114)	$ 20,145	7,748	$ 5,275	19,031	13,023		61,100	122,100	$ 210,100
Operating cash flows								3,069	2,218		(16,500)	(76,900)	(152,400)
Purchases of property and equipment								$ 2,840	$ 2,824
Repayment of outstanding debt		$ 28,900	$ 41,400
Decline in net revenue, as a percent								11.00%	31.00%
Expected annual savings	$ 8,000
Reduction in product, technology, general and administrative expenses				$ 2,600		$ 16,000		$ 7,600	$ 26,300
Reduction in product, technology, general and administrative expenses, as a percent				7.00%		31.00%		10.00%	26.00%
Reduction in capital expenditures										$ 3,000
Reduction in capital expenditures, as a percent										36.00%
Net cash used in operating activities								$ 3,069	$ 2,218		(16,500)	$ (76,900)	$ (152,400)
Revision of Prior Period, Accounting Standards Update, Adjustment [Member]
Significant Accounting Policies [Line Items]
Operating cash flows									(300)
Net cash used in operating activities									$ (300)
Letter of credit
Significant Accounting Policies [Line Items]
Outstanding borrowings in long term debt				$ 300				300		$ 300	300
Revolving credit facility
Significant Accounting Policies [Line Items]
Outstanding borrowings in long term debt				54,678				54,678		54,678	54,678
Remaining amount available to borrow				$ 0				$ 0		$ 0	$ 0