Related Party Transactions and Interests in Consolidated Entities - Amount of Quarterly Distribution (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	0 Months Ended			1 Months Ended		0 Months Ended			1 Months Ended
	May 06, 2013	Feb. 08, 2013	Aug. 02, 2012	Feb. 28, 2013	Nov. 30, 2012	Nov. 09, 2012	May 08, 2012	Feb. 10, 2012	May 30, 2013	Aug. 31, 2012	May 30, 2012	Feb. 29, 2012
Related Party Transaction [Line Items]
Distribution Declaration Date	May 06, 2013	Feb. 08, 2013	Aug. 02, 2012			Nov. 09, 2012	May 08, 2012	Feb. 10, 2012
Distributions per Class A Share Amount	$ 0.57	$ 1.05	$ 0.24			$ 0.40	$ 0.25	$ 0.46
Distributions Payment Date				Feb. 28, 2013	Nov. 30, 2012				May 30, 2013	Aug. 31, 2012	May 30, 2012	Feb. 29, 2012
Distributions to AGM Class A Shareholders				$ 390.7	$ 148.0				$ 212.6	$ 88.8	$ 91.6	$ 168.5
Distribution Equivalents on Participating Securities				25.0	9.4				14.3	5.3	6.2	10.3
Non-Controlling Interests in Consolidated Entities [Member]
Related Party Transaction [Line Items]
Distributions to AGM Class A Shareholders				252.0	96.0				131.8	57.6	60.0	110.4
Class A Shares [Member]
Related Party Transaction [Line Items]
Distributions to AGM Class A Shareholders				$ 138.7	$ 52.0				$ 80.8	$ 31.2	$ 31.6	$ 58.1