Investments - Schedule of Equity Method Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Equity method investments	$ 1,218.4	$ 950.9
Corporate Private Equity
Schedule of Investments [Line Items]
Equity method investments	369.5	282.4
Real Assets
Schedule of Investments [Line Items]
Equity method investments	775.1	622.8
Global Credit
Schedule of Investments [Line Items]
Equity method investments	23.0	20.1
Investment Solutions
Schedule of Investments [Line Items]
Equity method investments	$ 50.8	$ 25.6

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_EquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant