OTHER COMPREHENSIVE INCOME/(LOSS) - Accumulated Other Comprehensive Income (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) [Line Items]
Currency translation differences on investments in foreign subsidiaries	$ (108.7)	$ 167.4	$ (73.0)	$ (61.4)
Other comprehensive income, net	1.2	0.7	0.2	0.8
Other comprehensive income/(loss)	(107.5)	168.1	(72.8)	(60.6)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	15,426.6	13,967.7	14,808.9	14,318.3
Other comprehensive income/(loss), net of tax	(107.5)	168.1	(72.8)	(60.6)
Ending balance	15,732.0	14,350.6	15,732.0	14,350.6
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	(108.7)	167.4	(73.0)	(61.4)
Other comprehensive income, net	0.0	0.0	0.0	0.0
Other comprehensive income/(loss)	(108.7)	167.4	(73.0)	(61.4)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(243.6)	(690.8)	(279.3)	(462.0)
Other comprehensive income/(loss), net of tax	(108.7)	167.4	(73.0)	(61.4)
Ending balance	(352.3)	(523.4)	(352.3)	(523.4)
Employee benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	0.0	0.0	0.0	0.0
Other comprehensive income, net	1.2	0.7	0.3	0.9
Other comprehensive income/(loss)	1.2	0.7	0.3	0.9
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(126.9)	(125.9)	(126.0)	(126.1)
Other comprehensive income/(loss), net of tax	1.2	0.7	0.3	0.9
Ending balance	(125.7)	(125.2)	(125.7)	(125.2)
Equity method investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	0.0	0.0	0.0	0.0
Other comprehensive income, net	0.0	0.0	0.0	0.0
Other comprehensive income/(loss)	0.0	0.0	0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.1	0.1	0.1	0.1
Other comprehensive income/(loss), net of tax	0.0	0.0	0.0	0.0
Ending balance	0.1	0.1	0.1	0.1
Available-for-sale investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	0.0	0.0	0.0	0.0
Other comprehensive income, net	0.0	0.0	(0.1)	(0.1)
Other comprehensive income/(loss)	0.0	0.0	(0.1)	(0.1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.6	0.6	0.7	0.7
Other comprehensive income/(loss), net of tax	0.0	0.0	(0.1)	(0.1)
Ending balance	0.6	0.6	0.6	0.6
Total
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income/(loss)	(107.5)	168.1	(72.8)	(60.6)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(369.8)	(816.0)	(404.5)	(587.3)
Other comprehensive income/(loss), net of tax	(107.5)	168.1	(72.8)	(60.6)
Ending balance	$ (477.3)	$ (647.9)	$ (477.3)	$ (647.9)