Acquisitions (Details) (USD $) In Millions, unless otherwise specified			4 Months Ended		3 Months Ended	1 Months Ended
	Dec. 31, 2011	Mar. 31, 2011	Oct. 31, 2010 SunAmerica [Member]	Oct. 02, 2010 SunAmerica [Member]	Mar. 31, 2011 AXA [Member]	May 31, 2011 Pyrander Capital Management [Member]	Nov. 30, 2011 TIIM [Member]
Business Acquisition [Line Items]
Date of Acquisition			October 1, 2010			May 31, 2011
Purchase price of acquisition				$ 49.0	$ 21.4	$ 10.2
Cash Paid	1,500.0			14.3	21.4	5.5	15.0
Assumed deferred compensation liabilities				2.5
Net contingent consideration payable				32.2
Contingent consideration payable, recurring payment			1.5
Projected revenue sharing payments, contingent consideration payable			35.5
Performance-based fees			4.1
Goodwill	3,000.0			46.1		5.7	9.8
Ownership interest disposed (in hundredths)					50.00%
Business acquisition interest acquired (in hundredths)		50.00%			50.00%
Eliminated non-controlling interests					32.1
Increased partner's capital attributable to AllianceBernstein unitholders					10.7
Contingently liable to Caxton						4.4
Term of future purchase price payable (in years)						2Y
Miscellaneous liability assumed in purchase price						0.3
Indefinite-lived intangible assets acquired						2.5
Definite-lived intangible assets acquired						2.0
Number of restricted Holding unit awards						2
Term of first restricted Holding unit award (in years)						3Y
Term of second restricted Holding unit award (in years)						5Y
Contingently liability to Caxton						$ 4.4