STOCKHOLDERS' EQUITY - Accumulated Other Comprehensive Income (Loss) Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (200.0)	$ (294.8)	$ (208.4)	$ (257.5)
Other comprehensive income (loss) before reclassifications	(24.7)	40.6	(16.6)	(1.0)
Amounts reclassified from AOCI	(4.6)	0.3	(4.3)	4.6
Net other comprehensive income (loss)	(29.3)	40.9	(20.9)	3.6
Ending balance	(229.3)	(253.9)	(229.3)	(253.9)
CTA
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(156.6)	(243.5)	(145.2)	(208.2)
Other comprehensive income (loss) before reclassifications	(27.1)	45.0	(38.5)	9.7
Amounts reclassified from AOCI	0.0	0.0	0.0	0.0
Net other comprehensive income (loss)	(27.1)	45.0	(38.5)	9.7
Ending balance	(183.7)	(198.5)	(183.7)	(198.5)
Derivative Hedging Adj.
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	13.7	(7.1)	(6.0)	(0.8)
Other comprehensive income (loss) before reclassifications	1.1	(2.7)	21.6	(12.5)
Amounts reclassified from AOCI	(5.0)	(0.1)	(5.8)	3.4
Net other comprehensive income (loss)	(3.9)	(2.8)	15.8	(9.1)
Ending balance	9.8	(9.9)	9.8	(9.9)
Debt & Equity Securities Adj.
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0.6	3.6	1.2	2.6
Other comprehensive income (loss) before reclassifications	(0.2)	0.3	(0.8)	1.3
Amounts reclassified from AOCI	0.0	0.0	0.0	0.0
Net other comprehensive income (loss)	(0.2)	0.3	(0.8)	1.3
Ending balance	0.4	3.9	0.4	3.9
Pension Liability Adj.
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(57.7)	(47.8)	(58.4)	(51.1)
Other comprehensive income (loss) before reclassifications	1.5	(2.0)	1.1	0.5
Amounts reclassified from AOCI	0.4	0.4	1.5	1.2
Net other comprehensive income (loss)	1.9	(1.6)	2.6	1.7
Ending balance	$ (55.8)	$ (49.4)	$ (55.8)	$ (49.4)