Intangible Assets and Goodwill (Details 1) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 01, 2017
Goodwill [Line Items]
Elimination of goodwill allocated to our hemophilia business		$ 0.0
Income tax benefit (expense)	$ 2,458.7	1,237.3	$ 1,161.6
Accumulated impairment losses related to goodwill	0.0
Summary of roll forward of the changes in goodwill
Goodwill, beginning of period	3,669.3	2,663.8
Increase to goodwill	1,267.3	1,026.9
Other	10.0	(21.4)
Goodwill, end of period	4,632.5	3,669.3	$ 2,663.8
Fumapharm AG | TECFIDERA
Goodwill [Line Items]
Income tax benefit (expense)	232.7	173.1
Summary of roll forward of the changes in goodwill
Increase to goodwill	$ 1,500.0	$ 1,200.0
Bioverativ
Goodwill [Line Items]
Elimination of goodwill allocated to our hemophilia business				$ (314.1)