Earnings Per Share - Additional Information (Detail) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
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	164,341	491,856	82,165	302,457
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	1,153	0	579	0
Holdings Units
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Shares exchanged for Class A common stock			1
Class B Common Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Shares exchanged for Class A common stock			1
Class B Common Stock | Equity Unit Purchase Agreements
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Anti-dilutive securities excluded from the calculation of earnings per share	10,704,794	19,197,461	10,828,471	26,745,818