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Stock repurchase program, number of shares authorized to be repurchased (in shares) Stock Repurchase Program, Number of Shares Authorized to be Repurchased Biogen Research and License Agreement Biogen Research and License Agreement [Member] Biogen license agreement Share-Based Payment Arrangement, Tranche One Share-Based Payment Arrangement, Tranche One [Member] Tax Credit Carryforward Tax Credit Carryforward [Axis] Collaborative Arrangement [Member]