Investments - Investments Held by Consolidated Funds (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investment [Line Items]
Percentage of investments	100.00%	100.00%
Total loans	$ 13,656.7	$ 9,689.9
Total assets of the CLOs	3,000.0
Investments	24,836.3	19,595.4
Total investments	24,815.7	19,507.3
United States [Member]
Investment [Line Items]
Percentage of equity securities	6.95%	7.59%
Percentage of loans	1.35%	1.73%
Percentage of assets of the CLOs	35.19%	36.83%
Percentage of investments	66.93%	69.89%
Equity securities	1,726.0	1,482.3
Total loans	335.4	336.6
Total assets of the CLOs	8,734.5	7,184.7
Investments	16,610.0	13,633.7
United States [Member] | Accommodation and Food Services [Member]
Investment [Line Items]
Percentage of equity securities	0.08%	0.54%
Equity securities	20.2	106.1
United States [Member] | Administrative Support, Waste Management, Remediation Services [Member]
Investment [Line Items]
Percentage of equity securities	1.03%	0.02%
Percentage of loans	0.12%	0.31%
Equity securities	255.9	3.7
Total loans	31.0	60.6
United States [Member] | Aerospace and Defense [Member]
Investment [Line Items]
Percentage of equity securities		0.27%
Equity securities		53.2
United States [Member] | Finance and Insurance [Member]
Investment [Line Items]
Percentage of equity securities	0.31%	0.23%
Percentage of loans	0.18%
Equity securities	75.9	44.3
Total loans	44.2
United States [Member] | Health Care and Social Assistance [Member]
Investment [Line Items]
Percentage of equity securities	0.17%	0.16%
Equity securities	42.1	31.8
United States [Member] | Information [Member]
Investment [Line Items]
Percentage of equity securities	0.31%	0.25%
Equity securities	76.5	48.0
United States [Member] | Manufacturing [Member]
Investment [Line Items]
Percentage of equity securities	1.37%	2.12%
Percentage of loans	0.09%	0.33%
Equity securities	340.4	412.7
Total loans	21.2	65.0
United States [Member] | Professional, Scientific, Technical Services [Member]
Investment [Line Items]
Percentage of equity securities	2.46%	2.56%
Percentage of loans	0.43%	0.42%
Equity securities	610.9	500.0
Total loans	107.7	81.1
United States [Member] | Retail Trade [Member]
Investment [Line Items]
Percentage of equity securities	0.51%	0.75%
Equity securities	127.7	147.1
United States [Member] | Wholesale Trade [Member]
Investment [Line Items]
Percentage of equity securities	0.47%	0.09%
Percentage of loans	0.29%	0.25%
Equity securities	116.6	17.9
Total loans	72.0	48.2
United States [Member] | Other Equity Securities [Member]
Investment [Line Items]
Percentage of equity securities	0.24%	0.60%
Percentage of loans	0.24%	0.42%
Equity securities	59.8	117.5
Total loans	59.3	81.7
United States [Member] | Fund Investments [Member]
Investment [Line Items]
Total Partnership and LLC interests	2,925.4	2,701.0
Percentage of Partnership and LLC Interests	11.79%	13.85%
Percentage of investments	11.65%	9.89%
Investment in hedge funds	2,888.7	1,929.1
United States [Member] | Bond [Member]
Investment [Line Items]
Percentage of assets of the CLOs	1.17%	1.27%
Total assets of the CLOs	290.8	247.7
United States [Member] | Equity Securities [Member]
Investment [Line Items]
Percentage of assets of the CLOs	0.14%	0.13%
Total assets of the CLOs	35.0	25.3
United States [Member] | Loans [Member]
Investment [Line Items]
Percentage of assets of the CLOs	33.88%	35.43%
Total assets of the CLOs	8,408.7	6,911.6
United States [Member] | Other [Member]
Investment [Line Items]
Percentage of assets of the CLOs		0.00%
Total assets of the CLOs		0.1
Canada [Member]
Investment [Line Items]
Percentage of equity securities	0.01%	0.03%
Percentage of assets of the CLOs	0.84%	1.25%
Percentage of investments	0.99%	1.55%
Equity securities	2.1	5.8
Total assets of the CLOs	208.4	244.3
Total investments	244.9	303.1
Canada [Member] | Manufacturing [Member]
Investment [Line Items]
Percentage of equity securities	0.01%	0.01%
Equity securities	2.1	2.9
Canada [Member] | Fund Investments [Member]
Investment [Line Items]
Total Partnership and LLC interests	34.4	45.0
Percentage of Partnership and LLC Interests	0.14%	0.23%
Canada [Member] | Bond [Member]
Investment [Line Items]
Percentage of assets of the CLOs	0.01%	0.08%
Total assets of the CLOs	3.5	15.8
Canada [Member] | Loans [Member]
Investment [Line Items]
Percentage of assets of the CLOs	0.83%	1.17%
Total assets of the CLOs	204.9	228.5
Canada [Member] | Transportation And Warehousing [Member]
Investment [Line Items]
Percentage of equity securities		0.02%
Percentage of loans		0.04%
Equity securities		2.9
Total loans		8.0
Europe [Member]
Investment [Line Items]
Percentage of equity securities	5.69%	4.34%
Percentage of loans	0.01%	1.50%
Percentage of assets of the CLOs	19.60%	14.86%
Percentage of investments	28.92%	25.71%
Equity securities	1,411.8	846.8
Total loans	1.8	293.3
Total assets of the CLOs	4,863.8	2,899.0
Total investments	7,175.6	5,016.0
Europe [Member] | Administrative Support, Waste Management, Remediation Services [Member]
Investment [Line Items]
Percentage of equity securities	0.42%	0.53%
Equity securities	104.2	104.4
Europe [Member] | Health Care and Social Assistance [Member]
Investment [Line Items]
Percentage of equity securities	0.35%	0.07%
Equity securities	85.7	13.2
Europe [Member] | Information [Member]
Investment [Line Items]
Percentage of equity securities	0.40%	0.45%
Equity securities	98.6	88.1
Europe [Member] | Manufacturing [Member]
Investment [Line Items]
Percentage of equity securities	1.91%	2.00%
Percentage of loans	0.01%	0.81%
Equity securities	474.9	389.2
Total loans	1.8	158.2
Europe [Member] | Retail Trade [Member]
Investment [Line Items]
Percentage of equity securities	0.94%	0.49%
Equity securities	233.9	95.4
Europe [Member] | Wholesale Trade [Member]
Investment [Line Items]
Percentage of equity securities	0.44%	0.32%
Equity securities	109.3	62.8
Europe [Member] | Other Equity Securities [Member]
Investment [Line Items]
Percentage of equity securities	1.23%	0.48%
Equity securities	305.2	93.7
Europe [Member] | Fund Investments [Member]
Investment [Line Items]
Total Partnership and LLC interests	898.2	976.9
Percentage of Partnership and LLC Interests	3.62%	5.01%
Europe [Member] | Bond [Member]
Investment [Line Items]
Percentage of assets of the CLOs	2.53%	1.48%
Total assets of the CLOs	628.7	288.6
Europe [Member] | Equity Securities [Member]
Investment [Line Items]
Percentage of assets of the CLOs	0.00%	0.06%
Total assets of the CLOs	0.3	12.5
Europe [Member] | Loans [Member]
Investment [Line Items]
Percentage of assets of the CLOs	17.04%	13.21%
Total assets of the CLOs	4,227.5	2,577.2
Europe [Member] | Other [Member]
Investment [Line Items]
Percentage of loans		0.69%
Total loans		135.1
Europe [Member] | Other Loans [Member]
Investment [Line Items]
Percentage of assets of the CLOs	0.03%	0.11%
Equity securities	7.3	20.7
Australia [Member]
Investment [Line Items]
Percentage of assets of the CLOs	0.31%	0.03%
Percentage of investments	0.31%	0.03%
Total assets of the CLOs	76.3	4.9
Total investments	76.3	4.9
Australia [Member] | Bond [Member]
Investment [Line Items]
Percentage of assets of the CLOs	0.05%	0.03%
Total assets of the CLOs	11.2	4.9
Australia [Member] | Loans [Member]
Investment [Line Items]
Percentage of assets of the CLOs	0.26%
Total assets of the CLOs	65.1
Global [Member]
Investment [Line Items]
Total Partnership and LLC interests	457.5	475.7
Percentage of investments	2.85%	2.82%
Total investments	708.9	549.6
Global [Member] | Manufacturing [Member]
Investment [Line Items]
Percentage of equity securities	0.82%	0.38%
Equity securities	204.7	73.9
Global [Member] | Fund Investments [Member]
Investment [Line Items]
Percentage of Partnership and LLC Interests	1.84%	2.44%
Global [Member] | Loans [Member]
Investment [Line Items]
Percentage of equity securities	0.19%
Total assets of the CLOs	$ 46.7