Stockholders' Equity - Schedule of Shares Repurchased Under the Share Repurchase Program (Detail) - $ / shares	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Share Repurchase Program [Abstract]
Purchase of Class A common stock (in shares)	5,797,789	5,838,520
Average Price Per Share	$ 52.88	$ 43.25

Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration