Stockholders' equity (Additional Information) (Details)		12 Months Ended
	Mar. 21, 2017 shares	Dec. 31, 2019 $ / shares shares	Dec. 31, 2018 $ / shares	Apr. 12, 2010 class_of_stock stockholder $ / shares shares
Equity [Line Items]
Number of classes of common stock | class_of_stock				7
Stock par value (in dollars per share) | $ / shares		$ 0.01	$ 0.01	$ 0.01
Class G Common Stock
Equity [Line Items]
Common stock issued to former stockholders (in shares)				1
Number of common stockholders | stockholder				2
Class A Common Stock
Equity [Line Items]
Common stock shares issued (in shares)	37,110,630	84,347
Class B Common Stock
Equity [Line Items]
Common stock shares issued (in shares)	7,871,512
Common Stock
Equity [Line Items]
Common stock shares issued (in shares)	44,982,142