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Performance-Based Common Stock Warrants - Narrative (Details)
 12 Months Ended
Mar. 09, 2016
USD ($)
vesting_period
$ / shares
shares
Dec. 31, 2018
USD ($)
shares
Class of Warrant or Right [Line Items]    
Aggregate unrecognized estimated fair value of unvested warrants | $   $ 1,500,000
Common Stock Purchase Warrants    
Class of Warrant or Right [Line Items]    
Number of shares called by warrants (in shares) 725,000  
Exercise price of warrants (in dollars per share) | $ / shares $ 54.55  
Number of vesting periods | vesting_period 3  
Term of successive vesting periods 2 years  
Vesting period one 2 years  
Number of warrants vested (in shares)   175,000
Unearned underlying shares (in shares)   475,000
Remaining vesting period 4 years  
Number of warrants vested in the second period (in shares)   0
Vesting period two   2 years
Common Stock Purchase Warrants | Supply Threshold    
Class of Warrant or Right [Line Items]    
Aggregate level of purchases, $260 million threshold | $ $ 260,000,000  
Aggregate level of purchases, $340 million threshold | $ 340,000,000  
Supply threshold amount | $ $ 680,000,000  
Maximum potential warrants that will vest (in shares) 250,000