STOCKHOLDERS' EQUITY - Accumulated Other Comprehensive Income (Loss) Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (313.7)	$ (287.1)	$ (257.5)	$ (284.8)
Other comprehensive income (loss) before reclassifications	17.0	(2.2)	(41.7)	(6.8)
Amounts reclassified from AOCI	1.9	2.0	4.4	4.3
Net other comprehensive income (loss)	18.9	(0.2)	(37.3)	(2.5)
Ending balance	(294.8)	(287.3)	(294.8)	(287.3)
CTA
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(263.0)	(227.7)	(208.2)	(225.6)
Other comprehensive income (loss) before reclassifications	19.5	(4.0)	(35.3)	(6.1)
Amounts reclassified from AOCI	0.0	0.0	0.0	0.0
Net other comprehensive income (loss)	19.5	(4.0)	(35.3)	(6.1)
Ending balance	(243.5)	(231.7)	(243.5)	(231.7)
Derivative Hedging Adj.
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(5.0)	(5.5)	(0.8)	(4.4)
Other comprehensive income (loss) before reclassifications	(3.6)	0.6	(9.9)	(2.2)
Amounts reclassified from AOCI	1.5	1.4	3.6	3.1
Net other comprehensive income (loss)	(2.1)	2.0	(6.3)	0.9
Ending balance	(7.1)	(3.5)	(7.1)	(3.5)
Debt & Equity Securities Adj.
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	2.4	1.6	2.6	0.8
Other comprehensive income (loss) before reclassifications	1.2	0.5	1.0	1.3
Amounts reclassified from AOCI	0.0	0.0	0.0	0.0
Net other comprehensive income (loss)	1.2	0.5	1.0	1.3
Ending balance	3.6	2.1	3.6	2.1
Pension Liability Adj.
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(48.1)	(55.5)	(51.1)	(55.6)
Other comprehensive income (loss) before reclassifications	(0.1)	0.7	2.5	0.2
Amounts reclassified from AOCI	0.4	0.6	0.8	1.2
Net other comprehensive income (loss)	0.3	1.3	3.3	1.4
Ending balance	$ (47.8)	$ (54.2)	$ (47.8)	$ (54.2)