Incentive Plans - Summary of Activity Related to Shares of Restricted Class A Common Stock (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Class A Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs exchanged for shares of restricted Class A common stock	1,523,642
Restricted stock units exchanged for shares of restricted Class A common stock, grant date fair value	$ 35.95
RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs exchanged for shares of restricted Class A common stock	1,523,642