Condensed Consolidated Statements of Comprehensive Income Statement - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 489.2	$ 557.6
Other comprehensive income (loss), net of tax:
Foreign currency translation	(13.1)	(8.0)
Unrealized losses on debt securities	0.0	(0.2)
Unrealized gains (losses) on hedging activities, net of tax expense (benefit) of $15.8 and $(8.2), respectively	52.5	(26.5)
Other comprehensive income (loss), net of tax	39.4	(34.7)
Comprehensive income	528.6	522.9
Comprehensive loss attributable to noncontrolling interest	(146.8)	0.0
Comprehensive income attributable to Alexion	$ 675.4	$ 522.9