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Performance-Based Common Stock Warrants - Narrative (Details)
 12 Months Ended 24 Months Ended
Mar. 09, 2016
USD ($)
vesting_period
$ / shares
shares
Dec. 31, 2019
Dec. 31, 2017
Dec. 31, 2019
shares
Dec. 31, 2017
shares
Dec. 31, 2020
USD ($)
shares
Class of Warrant or Right [Line Items]            
Aggregate unrecognized estimated fair value of unvested warrants | $           $ 600,000
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 2 years 2 years      
Vesting period one 2 years          
Number of vested warrants outstanding (in shares)           275,000
Unearned underlying shares (in shares)           225,000
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 | $ $ 340,000,000          
Comcast | Common stock purchase warrants            
Class of Warrant or Right [Line Items]            
Number of warrants vested in period (in shares)       100,000 175,000  
Maximum warrants available to vest (in shares)       250,000 250,000