Commitments and Contingences (Details) - Fumapharm AG - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2006	Jun. 30, 2013
Business Acquisition, Contingent Consideration [Line Items]
Cash portion of consideration	$ 900.0	$ 220.0
TECFIDERA
Business Acquisition, Contingent Consideration [Line Items]
Maximum contingent consideration in the form of development and approval milestones			$ 15.0
Seven billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	7,000.0
Eight billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	8,000.0
Nine billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	9,000.0
Ten billion
Business Acquisition, Contingent Consideration [Line Items]
Cash portion of consideration	300.0
Cumulative sales level	10,000.0
Each additional one billion up to twenty billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	20,000.0
One billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	1,000.0
Three billion
Business Acquisition, Contingent Consideration [Line Items]
Cumulative sales level	$ 3,000.0