Stockholders' and Mezzanine equity - Equity Distribution Agreement (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	May 24, 2016
Common Stock
Proceeds from sale of common stock		$ 8,352	$ 1,056
Gross proceeds from sale of common stock		8,700
Professional services expenses		$ 300
Class A common stock
Common Stock
Offering value to sell common stock shares under equity distribution agreement				$ 73,000
Sale of common stock (in shares)	2.5	3.7
Proceeds from sale of common stock	$ 5,600
Gross proceeds from sale of common stock		$ 5,800
Professional services expenses	200
Aggregate offering price	$ 62,200	$ 62,200