Other Comprehensive Income/(Loss) - Accumulated Other Comprehensive Income (Details) £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 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	$ 0.8	$ (315.0)
Other comprehensive income, net	(4.0)	(3.2)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	14,808.9	14,318.3
Other comprehensive income/(loss)	(3.2)	(318.2)
Ending balance	15,029.5	13,942.4
Foreign currency transaction and translation gain (loss)	(5.7)	11.3
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Currency translation differences on investments in foreign subsidiaries	0.8	(315.0)
Other comprehensive income, net	0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(279.3)	(462.0)
Other comprehensive income/(loss)	0.8	(315.0)
Ending balance	(278.5)	(777.0)
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	(4.0)	(3.1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(126.0)	(126.1)
Other comprehensive income/(loss)	(4.0)	(3.1)
Ending balance	(130.0)	(129.2)
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	0.1
Other comprehensive income/(loss)	0.0	0.0
Ending balance	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
Other comprehensive income, net	0.0	(0.1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.7	0.7
Other comprehensive income/(loss)	0.0	(0.1)
Ending balance	0.7	0.6
Accumulated Other Comprehensive Income/(Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(404.5)	(587.3)
Other comprehensive income/(loss)	(3.2)	(318.2)
Ending balance	(407.7)	$ (905.5)
Designated as Hedging Instrument
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Intercompany debt	$ 180.2		£ 130	$ 174.5	£ 130