XML 31 R20.htm IDEA: XBRL DOCUMENT v3.5.0.2
Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - Biogen Inc [Member] - Change in estimated performance period [Member] - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2016
Change In Accounting Estimate [Line Items]    
Increase in net loss $ 0.9 $ 1.8
Increase in basic net loss per share $ 0.01 $ 0.03