Intangible Assets and Goodwill (Details 1) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Goodwill [Line Items]
Increase in goodwill			$ 810.7
Income tax expense (benefit)	$ 369.8	$ 383.8	956.0	$ 892.6
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period			4,632.5
Other			(3.1)
Goodwill, end of period	$ 5,440.1		5,440.1
TECFIDERA | Fumapharm AG
Goodwill [Line Items]
Increase in goodwill			900.0
Income tax expense (benefit)			$ 89.3