Taxation (Summary of (Provision) Benefit for Income Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal	$ 125.1	$ 102.0	$ 38.8
State	19.7	17.0	20.5
Foreign	35.5	94.0	158.2
Current income tax (expense)/benefit	180.3	213.0	217.5
Federal	57.6	16.6	35.5
State	5.2	4.3	10.0
Foreign	18.5	1.2	(8.0)
Deferred income tax (expense)/benefit	81.3	22.1	37.5
Total income tax expense (benefit)	$ 261.6	$ 235.1	$ 255.0