Stock-Based Incentive Plans - Movement in Outstanding Shares of Restricted Class A Common Stock and Weighted-Average Grant Date Fair Value Per Share (Detail) (Restricted Class A Common Stock [Member], USD $)	12 Months Ended
	Dec. 31, 2014
Restricted Class A Common Stock [Member]
Schedule Of Share Based Compensation Restricted Stock Units Award Activity [Line Items]
Restricted Class A Common Stock, Beginning Balance	700,000
Restricted Class A Common Stock, Granted	490,381
Restricted Class A Common Stock, Vested	(270,662)
Restricted Class A Common Stock, Ending Balance	919,719
Weighted-Average Grant Date Fair Value Per Share, Beginning Balance	$ 25.69
Weighted-Average Grant Date Fair Value Per Share, Granted	$ 27.82
Weighted-Average Grant Date Fair Value Per Share, Vested	$ 25.57
Weighted-Average Grant Date Fair Value Per Share, Ending Balance	$ 26.86