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Fair Value Measurements - Narrative (Details) - USD ($)
Dec. 31, 2019
Dec. 31, 2018
Mar. 31, 2017
Jun. 08, 2015
Non-financial Assets | Fair Value, Measurements, Nonrecurring        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Non-financial assets $ 0 $ 0    
1.00% Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt instrument, interest rate 1.00%      
1.625% Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt instrument, interest rate 1.625%      
Convertible Debt | 1.00% Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt instrument, interest rate 1.00% 1.00%   1.00%
Convertible Debt | 1.625% Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt instrument, interest rate 1.625% 1.625% 1.625%  
AXSEM        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Contingent consideration   $ 0