Net Income (Loss) Per Class A Share - Transactions in Class A Shares and Impact on Company's and Holdings' Ownership Interests (Detail) In Thousands, unless otherwise specified	12 Months Ended									6 Months Ended
	Dec. 31, 2012 January 18, 2012 [Member]	Dec. 31, 2012 February 13, 2012 [Member]	Dec. 31, 2012 March 5, 2012 [Member]	Dec. 31, 2012 April 3, 2012 [Member]	Dec. 31, 2012 July 9, 2012 [Member]	Dec. 31, 2012 August 6, 2012 [Member]	Dec. 31, 2012 October 9, 2012 [Member]	Dec. 31, 2012 November 12, 2012 [Member]	Dec. 31, 2012 November 19, 2012 [Member]	Jun. 30, 2013 January 9, 2013 [Member]	Jun. 30, 2013 January 25, 2013 [Member]	Jun. 30, 2013 February 11, 2013 [Member]	Jun. 30, 2013 March 19, 2013 [Member]	Jun. 30, 2013 April 9, 2013 [Member]	Jun. 30, 2013 May 9, 2013 [Member]	Jun. 30, 2013 May 14, 2013 [Member]
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Type of AGM Class A Shares Transaction	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance	Issuance/ Offering
Number of Shares Issued in AGM Class A Shares Transaction	394	1,994	50	150	1,452	1,962	150	25	5	150	23	1,913	5	150	627	8,800
AGM ownership % in AOG before AGM Class A Shares Transaction	34.10%	34.10%			34.50%	34.70%				35.10%		35.20%			35.50%	35.60%
AGM ownership % in AOG after AGM Class A Shares Transaction	34.10%	34.50%			34.70%	35.10%				35.20%		35.50%			35.60%	38.00%
Holdings ownership % in AOG before AGM Class A Shares Transaction	65.90%	65.90%			65.50%	65.30%				64.90%		64.80%			64.50%	64.40%
Holdings ownership % in AOG after AGM Class A Shares Transaction	65.90%	65.50%			65.30%	64.90%				64.80%		64.50%			64.40%	62.00%