Incentive Plans - Summary of Activity Related to Shares of Restricted Class A Common Stock (Detail) (RSA [Member], USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
RSA [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units, Beginning Balance	1,972,609	95,332	95,332
Units, Granted/Exchanged	368,736	2,100,965	327,238
Units, Forfeited	(37,782)	(21,178)
Units, Vested	(1,728,509)	(202,510)	(327,238)
Units, Ending Balance	575,054	1,972,609	95,332
Weighted Average Grant Date Fair Value, Beginning Balance	$ 34.85	$ 37.63	$ 37.63
Weighted Average Grant Date Fair Value, Granted/Exchanged	$ 36.74	$ 34.34	$ 43.70
Weighted Average Grant Date Fair Value, Forfeited	$ 33.37	$ 29.51
Weighted Average Grant Date Fair Value, Vested	$ 36.00	$ 31.43	$ 43.70
Weighted Average Grant Date Fair Value, Ending Balance	$ 32.72	$ 34.85	$ 37.63