Subsequent Events - Eisai (Details) - Subsequent Event - Eisai	1 Months Ended		9 Months Ended
	Jan. 31, 2019	Oct. 31, 2017	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of future development costs related to Eisai	45.00%	100.00%	15.00%
U.S.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses		55.00%
E.U.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses		68.50%
JAPAN
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Biogen share of co-promotion profits or losses		20.00%