Earnings per share - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Options
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Anti-dilutive securities excluded from the calculation of earnings per share	143,006	489,133	208,452
Restricted Stock Units
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Anti-dilutive securities excluded from the calculation of earnings per share	131	1,829	0
Class B Common Stock | Continuing LLC Owners Exchange Agreement
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Anti-dilutive securities excluded from the calculation of earnings per share	10,275,077	19,483,737	55,305,992