Stockholders' and Mezzanine equity - Offering of Class A Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 12, 2016	Aug. 25, 2016	Sep. 12, 2016	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Common Stock
Proceeds from sale of common stock					$ 8,332	$ 65,548
Class A common stock
Common Stock
Shares issued				0	3,700,000
Common stock, par value (in dollars per share)				$ 0.001	$ 0.001		$ 0.001
Proceeds from sale of common stock				$ 8,400
Class A common stock | Common Stock Underwriting Agreement
Common Stock
Shares issued		21,000,000
Proceeds from sale of common stock			$ 64,000
Class A common stock | Over-Allotment Option
Common Stock
Shares issued	3,150,000