COMMITMENTS AND CONTINGENCIES (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
COMMITMENTS AND CONTINGENCIES
Unfunded capital commitments		$ 285,700,000	$ 535,300,000
Future minimum commitments
2018		26,849,000
2019		26,251,000
2020		22,032,000
2021		17,726,000
2022		19,451,000
Thereafter		51,969,000
Total		164,278,000
Performance Fees
Performance Fees
Performance fees subject to potential clawback provision		476,100,000	418,300,000
Performance fees subject to potential claw back provision that are reimbursable by professionals		370,000,000	323,900,000
General, administrative and other expense
Operating Leases
Rent expense		26,100,000	26,400,000	$ 18,500,000
EIF Management, LLC
COMMITMENTS AND CONTINGENCIES
Unfunded commitment related to acquisition			20,300,000
Increase (decrease) in contingent consideration liability		20,300,000
Kayne Anderson Capital Advisors L.P.
COMMITMENTS AND CONTINGENCIES
Unfunded capital commitments		$ 16,500,000	$ 89,200,000
ARCC | American Capital Ltd.
ARCC Fee Waiver
Maximum fees waived	$ 10,000,000
Term of fee waiver	30 months
Maximum amount shortfall not carry over	$ 10,000,000
Remaining term of fee waiver		21 months
Remaining fees waived		$ 70,000,000