Other Comprehensive Income/(Loss) - Accumulated Other Comprehensive Income (Details) £ in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	$ 34.9	$ 86.2	$ 35.7	$ (228.8)
Other comprehensive income, net	3.0	3.3	(1.0)	0.1
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	15,029.5	13,942.4	14,808.9	14,318.3
Other comprehensive income/(loss)	37.9	89.5	34.7	(228.7)
Ending balance	15,426.6	13,967.7	15,426.6	13,967.7
Foreign currency transaction and translation gain (loss)	(2.2)	(1.9)	(7.9)	9.4
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	34.9	86.2	35.7	(228.8)
Other comprehensive income, net	0.0	0.0	0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(278.5)	(777.0)	(279.3)	(462.0)
Other comprehensive income/(loss)	34.9	86.2	35.7	(228.8)
Ending balance	(243.6)	(690.8)	(243.6)	(690.8)
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	3.1	3.3	(0.9)	0.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(130.0)	(129.2)	(126.0)	(126.1)
Other comprehensive income/(loss)	3.1	3.3	(0.9)	0.2
Ending balance	(126.9)	(125.9)	(126.9)	(125.9)
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
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.1	0.1	0.1	0.1
Other comprehensive income/(loss)	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	(0.1)	0.0	(0.1)	(0.1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.7	0.6	0.7	0.7
Other comprehensive income/(loss)	(0.1)	0.0	(0.1)	(0.1)
Ending balance	0.6	0.6	0.6	0.6
Accumulated Other Comprehensive Income/(Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(407.7)	(905.5)	(404.5)	(587.3)
Other comprehensive income/(loss)	37.9	89.5	34.7	(228.7)
Ending balance	(369.8)	$ (816.0)	(369.8)	$ (816.0)
Designated as Hedging Instrument
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Intercompany debt	$ 182.4		$ 182.4		£ 130	$ 174.5	£ 130