Treasury Stock - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares	6,405,000			6,405,000		6,405,000
Treasury stock, cost	$ 475,184,000			$ 475,184,000		$ 475,184,000
Class A | 2005 Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Shares repurchased program expiration date		Apr. 29, 2018
Stock authorized to repurchase by board of directors	$ 800,000,000			$ 800,000,000
Treasury stock, shares	3,826,947			3,826,947
Treasury stock, shares repurchased	0		0	0	0
Shares repurchased, average price per share				$ 90.23
Treasury stock, cost	$ 345,300,000			$ 345,300,000
Class A | 2005 Share Repurchase Program | Board of Directors
Equity, Class of Treasury Stock [Line Items]
Shares repurchased program expiration date				Apr. 29, 2023