Other Comprehensive Income/(Loss) - Accumulated other comprehensive income (Details) £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	$ (315.0)	$ 60.9
Other comprehensive income, net	(3.2)	0.5
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	14,318.3	8,936.2
Other comprehensive income/(loss)	(318.2)	61.4
Ending balance	13,942.4	9,009.4
Foreign currency transaction and translation gain (loss)	11.3	(4.1)
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	(315.0)	60.9
Other comprehensive income, net	0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(462.0)	(617.6)
Other comprehensive income/(loss)	(315.0)	60.9
Ending balance	(777.0)	(556.7)
Employee benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	0.0	0.0
Other comprehensive income, net	(3.1)	0.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(126.1)	(117.7)
Other comprehensive income/(loss)	(3.1)	0.2
Ending balance	(129.2)	(117.5)
Equity method investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	0.0	0.0
Other comprehensive income, net	0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.1	0.0
Other comprehensive income/(loss)	0.0	0.0
Ending balance	0.1	0.0
Available-for-sale investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	0.0	0.0
Other comprehensive income, net	(0.1)	0.3
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.7	0.3
Other comprehensive income/(loss)	(0.1)	0.3
Ending balance	0.6	0.6
Accumulated Other Comprehensive Income/(Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(587.3)	(735.0)
Other comprehensive income/(loss)	(318.2)	61.4
Ending balance	(905.5)	$ (673.6)
Designated as Hedging Instrument
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Intercompany debt	$ 160.8		£ 130	$ 172.1	£ 130