Investments - Partnership Equity Method Investees, Summarized Statement of Income Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investments [Line Items]
Investment income	$ 1,401.3	$ 1,060.4	$ 1,354.2
Expenses	972.8	938.1	1,060.3
Net investment income (loss)	428.5	122.3	293.9
Net realized and unrealized gain	7,057.0	6,631.7	12,804.6
Net income	7,485.5	6,754.0	13,098.5
Corporate Private Equity [Member]
Schedule of Investments [Line Items]
Investment income	733.3	496.7	733.2
Expenses	526.0	497.7	582.8
Net investment income (loss)	207.3	(1.0)	150.4
Net realized and unrealized gain	5,401.9	4,320.7	9,911.3
Net income	5,609.2	4,319.7	10,061.7
Global Market Strategies [Member]
Schedule of Investments [Line Items]
Investment income	150.9	127.5	266.3
Expenses	65.3	37.5	42.3
Net investment income (loss)	85.6	90.0	224.0
Net realized and unrealized gain	297.1	79.3	529.1
Net income	382.7	169.3	753.1
Real Assets [Member]
Schedule of Investments [Line Items]
Investment income	517.1	436.2	354.7
Expenses	381.5	402.9	435.2
Net investment income (loss)	135.6	33.3	(80.5)
Net realized and unrealized gain	1,358.0	2,231.7	2,364.2
Net income	$ 1,493.6	$ 2,265.0	$ 2,283.7