INVESTMENTS (Investments of the Consolidated Funds) (Details) - Consolidated Funds $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) issuer	Dec. 31, 2016 USD ($)
Investments
Investments, at fair value	$ 5,582,842	$ 3,330,203
Number of single issuers above 5% | issuer	0
Single issuer or investor threshold, as a percent	5.00%
Fixed income asset
Investments
Investments, at fair value	$ 5,115,374	$ 2,953,564
Fair value as a percentage of total investments	91.60%	88.80%
Equity securities
Investments
Investments, at fair value	$ 235,136	$ 204,943
Fair value as a percentage of total investments	4.20%	6.00%
Partnership interests
Investments
Investments, at fair value	$ 232,332	$ 171,696
Fair value as a percentage of total investments	4.20%	5.20%
United States | Fixed income asset
Investments
Investments, at fair value	$ 3,447,953	$ 1,955,301
Fair value as a percentage of total investments	61.80%	58.70%
Investments, at cost	$ 3,459,318	$ 1,945,977
United States | Fixed income asset | Consumer discretionary
Investments
Investments, at fair value	$ 1,295,732	$ 665,773
Fair value as a percentage of total investments	23.20%	20.00%
United States | Fixed income asset | Consumer staples
Investments
Investments, at fair value	$ 55,073	$ 64,840
Fair value as a percentage of total investments	1.00%	1.90%
United States | Fixed income asset | Energy
Investments
Investments, at fair value	$ 176,836	$ 45,409
Fair value as a percentage of total investments	3.20%	1.40%
United States | Fixed income asset | Financials
Investments
Investments, at fair value	$ 270,520	$ 139,285
Fair value as a percentage of total investments	4.80%	4.20%
United States | Fixed income asset | Healthcare, education and childcare
Investments
Investments, at fair value	$ 449,888	$ 246,403
Fair value as a percentage of total investments	8.10%	7.40%
United States | Fixed income asset | Industrials
Investments
Investments, at fair value	$ 370,926	$ 149,632
Fair value as a percentage of total investments	6.60%	4.50%
United States | Fixed income asset | Information technology
Investments
Investments, at fair value	$ 167,089	$ 194,394
Fair value as a percentage of total investments	3.00%	5.80%
United States | Fixed income asset | Materials
Investments
Investments, at fair value	$ 185,170	$ 139,994
Fair value as a percentage of total investments	3.30%	4.20%
United States | Fixed income asset | Telecommunication services
Investments
Investments, at fair value	$ 399,617	$ 261,771
Fair value as a percentage of total investments	7.20%	7.90%
United States | Fixed income asset | Utilities
Investments
Investments, at fair value	$ 77,102	$ 47,800
Fair value as a percentage of total investments	1.40%	1.40%
United States | Equity securities
Investments
Investments, at fair value	$ 126	$ 421
Fair value as a percentage of total investments	0.00%	0.00%
Investments, at cost	$ 2,265	$ 2,872
United States | Equity securities | Energy
Investments
Investments, at fair value	$ 126	$ 421
Fair value as a percentage of total investments	0.00%	0.00%
United States | Partnership interests
Investments
Investments, at fair value	$ 232,332	$ 171,696
Fair value as a percentage of total investments	4.20%	5.20%
Investments, at cost	$ 190,000	$ 147,000
United States | Partnership interests | Partnership interests
Investments
Investments, at fair value	$ 232,332	$ 171,696
Fair value as a percentage of total investments	4.20%	5.20%
Europe | Fixed income asset
Investments
Investments, at fair value	$ 1,537,889	$ 869,474
Fair value as a percentage of total investments	27.60%	26.20%
Investments, at cost	$ 1,545,297	$ 892,108
Europe | Fixed income asset | Consumer discretionary
Investments
Investments, at fair value	$ 604,608	$ 274,678
Fair value as a percentage of total investments	10.80%	8.20%
Europe | Fixed income asset | Consumer staples
Investments
Investments, at fair value	$ 76,361	$ 39,197
Fair value as a percentage of total investments	1.40%	1.20%
Europe | Fixed income asset | Energy
Investments
Investments, at fair value	$ 2,413	$ 0
Fair value as a percentage of total investments	0.00%	0.00%
Europe | Fixed income asset | Financials
Investments
Investments, at fair value	$ 81,987	$ 28,769
Fair value as a percentage of total investments	1.50%	0.90%
Europe | Fixed income asset | Healthcare, education and childcare
Investments
Investments, at fair value	$ 209,569	$ 111,589
Fair value as a percentage of total investments	3.80%	3.40%
Europe | Fixed income asset | Industrials
Investments
Investments, at fair value	$ 145,706	$ 118,466
Fair value as a percentage of total investments	2.60%	3.60%
Europe | Fixed income asset | Information technology
Investments
Investments, at fair value	$ 21,307	$ 49,507
Fair value as a percentage of total investments	0.40%	1.50%
Europe | Fixed income asset | Materials
Investments
Investments, at fair value	$ 213,395	$ 124,629
Fair value as a percentage of total investments	3.80%	3.70%
Europe | Fixed income asset | Telecommunication services
Investments
Investments, at fair value	$ 182,543	$ 118,632
Fair value as a percentage of total investments	3.30%	3.60%
Europe | Fixed income asset | Utilities
Investments
Investments, at fair value	$ 0	$ 4,007
Fair value as a percentage of total investments	0.00%	0.10%
Europe | Equity securities
Investments
Investments, at fair value	$ 63,155	$ 42,870
Fair value as a percentage of total investments	1.10%	1.20%
Investments, at cost	$ 67,198	$ 67,290
Europe | Equity securities | Consumer staples
Investments
Investments, at fair value	$ 0	$ 1,517
Fair value as a percentage of total investments	0.00%	0.00%
Europe | Equity securities | Healthcare, education and childcare
Investments
Investments, at fair value	$ 63,155	$ 41,329
Fair value as a percentage of total investments	1.10%	1.20%
Europe | Equity securities | Telecommunication services
Investments
Investments, at fair value	$ 0	$ 24
Fair value as a percentage of total investments	0.00%	0.00%
Asia and other | Fixed income asset
Investments
Investments, at fair value	$ 36,309	$ 44,188
Fair value as a percentage of total investments	0.60%	1.30%
Investments, at cost	$ 36,180	$ 46,545
Asia and other | Fixed income asset | Consumer discretionary
Investments
Investments, at fair value	$ 2,008	$ 24,244
Fair value as a percentage of total investments	0.00%	0.70%
Asia and other | Fixed income asset | Financials
Investments
Investments, at fair value	$ 12,453	$ 1,238
Fair value as a percentage of total investments	0.20%	0.00%
Asia and other | Fixed income asset | Healthcare, education and childcare
Investments
Investments, at fair value	$ 0	$ 10,010
Fair value as a percentage of total investments	0.00%	0.30%
Asia and other | Fixed income asset | Telecommunication services
Investments
Investments, at fair value	$ 21,848	$ 8,696
Fair value as a percentage of total investments	0.40%	0.30%
Asia and other | Equity securities
Investments
Investments, at fair value	$ 165,943	$ 143,919
Fair value as a percentage of total investments	3.00%	4.30%
Investments, at cost	$ 122,418	$ 122,418
Asia and other | Equity securities | Consumer discretionary
Investments
Investments, at fair value	$ 59,630	$ 44,642
Fair value as a percentage of total investments	1.10%	1.30%
Asia and other | Equity securities | Consumer staples
Investments
Investments, at fair value	$ 45,098	$ 50,101
Fair value as a percentage of total investments	0.80%	1.50%
Asia and other | Equity securities | Healthcare, education and childcare
Investments
Investments, at fair value	$ 44,637	$ 32,598
Fair value as a percentage of total investments	0.80%	1.00%
Asia and other | Equity securities | Industrials
Investments
Investments, at fair value	$ 16,578	$ 16,578
Fair value as a percentage of total investments	0.30%	0.50%
Canada | Fixed income asset
Investments
Investments, at fair value	$ 80,797	$ 48,848
Fair value as a percentage of total investments	1.40%	1.50%
Investments, at cost	$ 80,201	$ 48,274
Canada | Fixed income asset | Consumer discretionary
Investments
Investments, at fair value	$ 6,757	$ 0
Fair value as a percentage of total investments	0.10%	0.00%
Canada | Fixed income asset | Consumer staples
Investments
Investments, at fair value	$ 15,351	$ 5,256
Fair value as a percentage of total investments	0.30%	0.20%
Canada | Fixed income asset | Energy
Investments
Investments, at fair value	$ 33,715	$ 12,830
Fair value as a percentage of total investments	0.60%	0.40%
Canada | Fixed income asset | Healthcare, education and childcare
Investments
Investments, at fair value	$ 0	$ 15,509
Fair value as a percentage of total investments	0.00%	0.50%
Canada | Fixed income asset | Industrials
Investments
Investments, at fair value	$ 18,785	$ 1,401
Fair value as a percentage of total investments	0.30%	0.00%
Canada | Fixed income asset | Telecommunication services
Investments
Investments, at fair value	$ 6,189	$ 13,852
Fair value as a percentage of total investments	0.10%	0.40%
Canada | Equity securities
Investments
Investments, at fair value	$ 5,912	$ 164
Fair value as a percentage of total investments	0.10%	0.00%
Investments, at cost	$ 17,202	$ 408
Canada | Equity securities | Energy
Investments
Investments, at fair value	$ 5,912	$ 164
Fair value as a percentage of total investments	0.10%	0.00%
Australia | Fixed income asset
Investments
Investments, at fair value	$ 12,426	$ 35,753
Fair value as a percentage of total investments	0.20%	1.10%
Investments, at cost	$ 12,714	$ 37,975
Australia | Fixed income asset | Consumer discretionary
Investments
Investments, at fair value	$ 10,863	$ 5,627
Fair value as a percentage of total investments	0.20%	0.20%
Australia | Fixed income asset | Energy
Investments
Investments, at fair value	$ 1,563	$ 6,046
Fair value as a percentage of total investments	0.00%	0.20%
Australia | Fixed income asset | Industrials
Investments
Investments, at fair value	$ 0	$ 2,926
Fair value as a percentage of total investments	0.00%	0.10%
Australia | Fixed income asset | Utilities
Investments
Investments, at fair value	$ 0	$ 21,154
Fair value as a percentage of total investments	0.00%	0.60%
Australia | Equity securities
Investments
Investments, at fair value	$ 0	$ 17,569
Fair value as a percentage of total investments	0.00%	0.50%
Investments, at cost	$ 0	$ 18,442
Australia | Equity securities | Utilities
Investments
Investments, at fair value	$ 0	$ 17,569
Fair value as a percentage of total investments	0.00%	0.50%