INVESTMENTS (Investments of the Consolidated Funds) (Details) - Consolidated Funds $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) issuer	Dec. 31, 2017 USD ($) issuer
Investments
Total investments	$ 6,968,067	$ 5,582,842
Number of single issuers above 5% | issuer	0	0
Single issuer or investor threshold, as a percent	5.00%	5.00%
Fixed Income
Investments
Total investments	$ 6,483,593	$ 5,115,374
Percentage of total investments	93.20%	91.60%
Equity securities
Investments
Total investments	$ 232,866	$ 235,136
Percentage of total investments	3.20%	4.20%
Partnership Interests | Partnership and interests
Investments
Total investments	$ 251,608	$ 232,332
Percentage of total investments	3.60%	4.20%
United States | Fixed Income
Investments
Total investments	$ 4,528,369	$ 3,447,953
Percentage of total investments	65.10%	61.80%
Investments, at cost	$ 4,573,566	$ 3,459,318
United States | Fixed Income | Consumer discretionary
Investments
Total investments	$ 1,714,080	$ 1,295,732
Percentage of total investments	24.80%	23.20%
United States | Fixed Income | Consumer staples
Investments
Total investments	$ 76,664	$ 55,073
Percentage of total investments	1.10%	1.00%
United States | Fixed Income | Energy
Investments
Total investments	$ 169,208	$ 176,836
Percentage of total investments	2.40%	3.20%
United States | Fixed Income | Financials
Investments
Total investments	$ 424,838	$ 270,520
Percentage of total investments	6.10%	4.80%
United States | Fixed Income | Healthcare, education and childcare
Investments
Total investments	$ 665,530	$ 449,888
Percentage of total investments	9.60%	8.10%
United States | Fixed Income | Industrials
Investments
Total investments	$ 407,280	$ 370,926
Percentage of total investments	5.80%	6.60%
United States | Fixed Income | Information technology
Investments
Total investments	$ 175,704	$ 167,089
Percentage of total investments	2.50%	3.00%
United States | Fixed Income | Materials
Investments
Total investments	$ 189,786	$ 185,170
Percentage of total investments	2.70%	3.30%
United States | Fixed Income | Telecommunication services
Investments
Total investments	$ 625,619	$ 399,617
Percentage of total investments	9.00%	7.20%
United States | Fixed Income | Utilities
Investments
Total investments	$ 79,660	$ 77,102
Percentage of total investments	1.10%	1.40%
United States | Equity securities
Investments
Total investments	$ 60	$ 126
Percentage of total investments	0.00%	0.00%
Investments, at cost	$ 2,265	$ 2,265
United States | Equity securities | Energy
Investments
Total investments	$ 60	$ 126
Percentage of total investments	0.00%	0.00%
United States | Partnership Interests
Investments
Total investments	$ 251,608	$ 232,332
Percentage of total investments	3.60%	4.20%
Investments, at cost	$ 206,000	$ 190,000
United States | Partnership Interests | Partnership and interests
Investments
Total investments	$ 251,608	$ 232,332
Percentage of total investments	3.60%	4.20%
Europe | Fixed Income
Investments
Total investments	$ 1,825,984	$ 1,537,889
Percentage of total investments	26.20%	27.60%
Investments, at cost	$ 1,849,235	$ 1,545,297
Europe | Fixed Income | Consumer discretionary
Investments
Total investments	$ 772,714	$ 604,608
Percentage of total investments	11.10%	10.80%
Europe | Fixed Income | Consumer staples
Investments
Total investments	$ 90,207	$ 76,361
Percentage of total investments	1.30%	1.40%
Europe | Fixed Income | Energy
Investments
Total investments	$ 14,833	$ 2,413
Percentage of total investments	0.20%	0.00%
Europe | Fixed Income | Financials
Investments
Total investments	$ 127,141	$ 81,987
Percentage of total investments	1.80%	1.50%
Europe | Fixed Income | Healthcare, education and childcare
Investments
Total investments	$ 234,696	$ 209,569
Percentage of total investments	3.40%	3.80%
Europe | Fixed Income | Industrials
Investments
Total investments	$ 124,690	$ 145,706
Percentage of total investments	1.80%	2.60%
Europe | Fixed Income | Information technology
Investments
Total investments	$ 21,329	$ 21,307
Percentage of total investments	0.30%	0.40%
Europe | Fixed Income | Materials
Investments
Total investments	$ 184,342	$ 213,395
Percentage of total investments	2.60%	3.80%
Europe | Fixed Income | Telecommunication services
Investments
Total investments	$ 256,032	$ 182,543
Percentage of total investments	3.70%	3.30%
Europe | Equity securities
Investments
Total investments	$ 51,010	$ 63,155
Percentage of total investments	0.70%	1.10%
Investments, at cost	$ 67,198	$ 67,198
Europe | Equity securities | Healthcare, education and childcare
Investments
Total investments	$ 51,010	$ 63,155
Percentage of total investments	0.70%	1.10%
Asia and other | Fixed Income
Investments
Total investments	$ 27,054	$ 36,309
Percentage of total investments	0.40%	0.60%
Investments, at cost	$ 27,737	$ 36,180
Asia and other | Fixed Income | Consumer discretionary
Investments
Total investments	$ 1,878	$ 2,008
Percentage of total investments	0.00%	0.00%
Asia and other | Fixed Income | Financials
Investments
Total investments	$ 4,288	$ 12,453
Percentage of total investments	0.10%	0.20%
Asia and other | Fixed Income | Telecommunication services
Investments
Total investments	$ 20,888	$ 21,848
Percentage of total investments	0.30%	0.40%
Asia and other | Equity securities
Investments
Total investments	$ 181,796	$ 165,943
Percentage of total investments	2.50%	3.00%
Investments, at cost	$ 122,418	$ 122,418
Asia and other | Equity securities | Consumer discretionary
Investments
Total investments	$ 43,647	$ 59,630
Percentage of total investments	0.60%	1.10%
Asia and other | Equity securities | Consumer staples
Investments
Total investments	$ 42,717	$ 45,098
Percentage of total investments	0.60%	0.80%
Asia and other | Equity securities | Healthcare, education and childcare
Investments
Total investments	$ 44,637	$ 44,637
Percentage of total investments	0.60%	0.80%
Asia and other | Equity securities | Industrials
Investments
Total investments	$ 50,795	$ 16,578
Percentage of total investments	0.70%	0.30%
Canada | Fixed Income
Investments
Total investments	$ 88,527	$ 80,797
Percentage of total investments	1.30%	1.40%
Investments, at cost	$ 89,165	$ 80,201
Canada | Fixed Income | Consumer discretionary
Investments
Total investments	$ 7,287	$ 6,757
Percentage of total investments	0.10%	0.10%
Canada | Fixed Income | Consumer staples
Investments
Total investments	$ 36,420	$ 15,351
Percentage of total investments	0.50%	0.30%
Canada | Fixed Income | Energy
Investments
Total investments	$ 4,895	$ 33,715
Percentage of total investments	0.10%	0.60%
Canada | Fixed Income | Industrials
Investments
Total investments	$ 27,356	$ 18,785
Percentage of total investments	0.40%	0.30%
Canada | Fixed Income | Telecommunication services
Investments
Total investments	$ 12,569	$ 6,189
Percentage of total investments	0.20%	0.10%
Canada | Equity securities
Investments
Total investments	$ 0	$ 5,912
Percentage of total investments	0.00%	0.10%
Investments, at cost	$ 0	$ 17,202
Canada | Equity securities | Energy
Investments
Total investments	$ 0	$ 5,912
Percentage of total investments	0.00%	0.10%
Australia | Fixed Income
Investments
Total investments	$ 13,659	$ 12,426
Percentage of total investments	0.20%	0.20%
Investments, at cost	$ 13,915	$ 12,714
Australia | Fixed Income | Consumer discretionary
Investments
Total investments	$ 11,932	$ 10,863
Percentage of total investments	0.20%	0.20%
Australia | Fixed Income | Energy
Investments
Total investments	$ 1,727	$ 1,563
Percentage of total investments	0.00%	0.00%