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THE COMPANY AND SIGNIFICANT ACCOUNTING POLICIES - Other Expense (Income) (Details) kr in Millions, $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 SEK (kr)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 SEK (kr)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
THE COMPANY AND SIGNIFICANT ACCOUNTING POLICIES
Derivative liability fair value adjustment for embedded conversion feature in the Convertible Notes due 2024				kr (23.5)	$ (23.5)	kr (66.4)	$ (66.4)
Derivative asset fair value adjustment for contingent call option related to the Class B common stock purchase and cancellation agreement				kr (17.7)	17.7	kr 45.0	(45.0)
Business interruption insurance recoveries					(1.1)		(0.4)	$ (0.4)
Loss on GBP forward contract					0.9		0.4
Foreign currency transactions (gain) losses					1.5		1.4	(3.0)
Non-operating components of net periodic benefit cost					1.2		0.8	0.2
Loss on repayment of indebtedness					16.6			0.4
Fees related to modification of term loans							0.4
Third party fees relating to Third Amendment to out Senior Secured Credit Agreement	$ 9.6	$ 28.4	$ 54.1					1.0
Other					0.1		0.7	0.3
Other expense (income)					$ 13.4		$ (108.1)	$ (1.5)