Earnings Per Common Share (Summary Of Calculation Of Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Common Share [Abstract]
Net income attributable to Alexion	$ 636.0	$ 557.6
Shares used in computing earnings per common share—basic (in shares)	220.1	221.6
Stock awards (in shares)	2.5	1.0
Shares used in computing earnings per common share-diluted (in shares)	222.6	222.6
Earnings per common share attributable to Alexion:
Basic (in dollars per share)	$ 2.89	$ 2.52
Diluted (in dollars per share)	$ 2.86	$ 2.50