Collaborative and Other Relationships Collaborative and Other Relationships - Collaborations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Jan. 31, 2007
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 437.3	$ 460.5
Sobi rate to Biogen on net sales in the Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method	Royalty
Base rate after first commercial sale	12.00%
Rate during the reimbursement period	Base Rate plus 5%
Biogen rate to Sobi on net sales in the Biogen North America Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method	Royalty
Rate during the reimbursement period	Base Rate less 5%
Biogen rate to Sobi on net sales in the Biogen North America Territory | Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Rate prior to first commercial sale	2.00%
Base rate after first commercial sale	12.00%
Biogen rate to Sobi on net sales in the Biogen Direct Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method	Royalty
Rate during the reimbursement period	Base Rate less 5%
Biogen rate to Sobi on net sales in the Biogen Direct Territory | Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Rate prior to first commercial sale	2.00%
Base rate after first commercial sale	17.00%
Biogen rate to Sobi on net revenue from the Biogen Distributor Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement under the amended agreement method	Net Revenue Share
Rate during the reimbursement period	Base Rate less 15%
Biogen rate to Sobi on net revenue from the Biogen Distributor Territory | Sobi Territory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Rate prior to first commercial sale	10.00%
Base rate after first commercial sale	50.00%
Biogen Hemophilia
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of stock acquired			100.00%
Swedish Orphan Biovitrum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period for granting of approval by approving authority	18 months
Reimbursement recognition period	10 years
Reimbursement cost achieving period	6 years
Time period for paying remaining balance due	90 days
Swedish Orphan Biovitrum | ELOCTATE
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 165.0
Swedish Orphan Biovitrum | ALPROLIX
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development	$ 135.0