SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective income tax rate (percent)	(7.10%)	174.40%	(47.90%)	194.80%
Decrease in valuation allowance		$ 111.2		$ 111.2
Tax Cuts and Jobs Act of 2017, one-time repatriation tax			$ 300.0
Tax Cuts and Jobs Act of 2017, revaluation of deferred taxes, income tax benefit			300.0
Gross unrecognized tax benefits	$ 260.6		260.6			$ 257.9
Unrecognized tax benefits that would impact effective tax rate	224.0		224.0
Unrecognized tax benefits, net	231.3		231.3			154.6
Interest and penalties expense	1.0	$ 0.8	2.1	1.0
Gross accrued interest and penalties	13.0		13.0			11.7
Reasonably possible decrease in UTBs (up to)	8.8		8.8
Adjustment due to the adoption of ASU 2016-09						8.3
Payments for employee taxes related to net settlement of equity awards (see Note 2)			3.4	$ 0.0
(Accumulated Deficit)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment due to the adoption of ASU 2016-09						$ 8.3
Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
U.S. federal statutory rate (percent)					28.00%
Foreign Tax Authority
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reduction of gross unrecognized tax benefits	43.0		43.0
Reduction in unrecognized tax benefits, income tax expense	41.8		41.8
Reduction in unrecognized tax benefits, interest	$ 1.2		$ 1.2