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Fair Value Measurements - Tysabri Financial Asset (Details) - Tysabri Royalty Stream - Recurring - Level 3 - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ 2,350.0 $ 5,310.0
Royalties earned 0.0 (351.8)
Change in fair value 0.0 (2,608.2)
Divestitures (2,350.0) 0.0
Ending balance $ 0.0 $ 2,350.0