Other Comprehensive Income and Accumulated Other Comprehensive Income (Significant Reclassifications from AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total revenues	$ 1,636.5	$ 1,444.8
Interest expense	27.1	25.8
Income before income taxes	602.6	663.6
Income tax provision	(113.4)	(106.0)
Net income attributable to Alexion	636.0	557.6
Unrealized Gains (Losses) from Hedging Activities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total revenues	(11.8)	11.4
Interest expense	(11.4)	(4.6)
Income before income taxes	(23.2)	6.8
Income tax provision	5.3	(1.6)
Net income attributable to Alexion	$ (17.9)	$ 5.2