Intangible Assets and Goodwill (Details 1) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Goodwill [Line Items]
Increase in goodwill	$ 270.5
Income tax expense (benefit)	322.5	$ 239.2
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	4,632.5
Other	4.8
Goodwill, end of period	4,907.8
TECFIDERA | Fumapharm AG
Goodwill [Line Items]
Increase in goodwill	300.0
Income tax expense (benefit)	$ 29.5