Contingent Consideration - Maximum Amounts of Cash Obligations with Business Acquisitions and Our Strategic Investment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	$ 989.2
Liability Recognized on Financial Statements	273.7
Performance-based [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	546.3
Liability Recognized on Financial Statements	166.8
Employment-based contingent cash consideration [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	345.7
Liability Recognized on Financial Statements	106.9
Options to acquire additional investments in NGP [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	97.2
Hedge Fund Acquisitions [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	664.0
Hedge Fund Acquisitions [Member] | Performance-based [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	363.3
Hedge Fund Acquisitions [Member] | Employment-based contingent cash consideration [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	300.7
NGP Investment [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	325.2
NGP Investment [Member] | Performance-based [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	183.0
NGP Investment [Member] | Employment-based contingent cash consideration [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	45.0
NGP Investment [Member] | Options to acquire additional investments in NGP [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent Cash Obligations	$ 97.2