Net Income (Loss) Per Class A Share - Transactions in Class A Shares and Impact on Company's and Holdings' Ownership Interests (Detail)	3 Months Ended							1 Months Ended	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013		Mar. 31, 2013	May 14, 2013 Common Class A	Dec. 31, 2013 Common Class A
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Type of AGM Class A Shares Transaction	Issuance	Issuance/Offering		Issuance	Issuance/Offering		Issuance
Number of Shares Issued in AGM Class A Shares Transaction	2,672,000	2,581,000		1,977,000	9,577,000		2,091,000
AGM ownership % in AOG before AGM Class A Shares Transaction	39.00%	38.30%		38.00%	35.50%		35.10%
AGM ownership % in AOG after AGM Class A Shares Transaction	39.40%	39.00%		38.30%	38.00%		35.50%
Holdings ownership % in AOG before AGM Class A Shares Transaction	61.00%	61.70%		62.00%	64.50%		64.90%
Holdings ownership % in AOG after AGM Class A Shares Transaction	60.60%	61.00%	[1]	61.70%	62.00%	[1]	64.50%
Shares issued for conversion								8,800,000
Stock Issued During Period, Shares, Conversion of Convertible Securities									2,300,000

[1] (1)Certain holders of AOG Units exchanged their AOG Units for Class A shares. Approximately 8.8 million Class A shares were issued by the Company in the exchange, which settled on May 14, 2013. See note 1 for details regarding the Secondary Offering. In November 2013, certain holders of AOG Units exchanged their AOG Units for Class A shares and approximately 2.3 million Class A shares were issued by the Company in the exchange.