Intangible Assets and Goodwill (Details 1) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Income tax benefit (expense)		$ 1,158.0	$ 1,425.6	$ 2,458.7
Accumulated impairment losses related to goodwill		0.0
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period		5,706.4	4,632.5
Increase to goodwill		117.5	1,080.1
Elimination of goodwill allocated to Hillerød, Denmark manufacturing operations		69.5	0.0
Other		3.4	(6.2)
Goodwill, end of period	$ 5,706.4	$ 5,757.8	5,706.4	$ 4,632.5
Fumapharm AG | TECFIDERA
Goodwill [Line Items]
Income tax benefit (expense)			119.9
Summary of roll forward of the changes in goodwill
Increase to goodwill	300.0		1,200.0
Each additional one billion up to twenty billion | Fumapharm AG | TECFIDERA
Goodwill [Line Items]
Cumulative sales level	$ 20,000.0		$ 20,000.0