Stockholders' and Mezzanine equity - Equity Distribution Agreement (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	May 24, 2016
Common Stock
Proceeds from sale of common stock		$ 8,332	$ 65,548
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)	0.0	3.7
Proceeds from sale of common stock	$ 8,400
Gross proceeds from sale of common stock		$ 8,700
Professional services expenses	300
Aggregate offering price	$ 62,200	$ 62,200