SEGMENT REPORTING - Segment Data (Details) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2019 segment	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	3
Number of operating segments | segment	3
Net revenues		$ 4,717.8	$ 6,287.9	$ 6,841.8
Depreciation and amortization		716.5	736.0	737.0
Operating (loss) income		(1,236.5)	(3,688.4)	(155.5)
Interest expense, net		242.7	225.2	200.6
Loss on early extinguishment of debt		0.0	0.0	10.7
Other (income) expense, net		(11.6)	31.8	27.7
Loss from continuing operations before income taxes		(1,467.6)	(3,945.4)	(394.5)
Continuing Operations
Segment Reporting Information [Line Items]
Depreciation and amortization		584.7	578.5	575.4
Americas
Segment Reporting Information [Line Items]
Net revenues		1,771.0	2,248.9	2,399.3
EMEA
Segment Reporting Information [Line Items]
Net revenues		2,308.6	2,909.7	3,250.7
Asia Pacific
Segment Reporting Information [Line Items]
Net revenues		582.7	771.1	758.7
Other
Segment Reporting Information [Line Items]
Net revenues		55.5	358.2	433.1
Operating Segments | Americas
Segment Reporting Information [Line Items]
Operating (loss) income		(164.8)	(1,474.5)	45.6
Operating Segments | Americas | Continuing Operations
Segment Reporting Information [Line Items]
Depreciation and amortization		227.4	188.4	192.6
Operating Segments | EMEA
Segment Reporting Information [Line Items]
Operating (loss) income		(248.4)	(1,344.1)	131.4
Operating Segments | EMEA | Continuing Operations
Segment Reporting Information [Line Items]
Depreciation and amortization		276.0	272.0	271.0
Operating Segments | Asia Pacific
Segment Reporting Information [Line Items]
Operating (loss) income		(74.0)	(253.1)	52.7
Operating Segments | Asia Pacific | Continuing Operations
Segment Reporting Information [Line Items]
Depreciation and amortization		69.5	63.3	56.3
Operating Segments | Other
Segment Reporting Information [Line Items]
Operating (loss) income		(10.9)	(18.6)	70.1
Operating Segments | Other | Continuing Operations
Segment Reporting Information [Line Items]
Depreciation and amortization		11.8	54.8	55.5
Corporate
Segment Reporting Information [Line Items]
Operating (loss) income		$ (738.4)	$ (598.1)	$ (455.3)