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Significant Accounting Policies - Fair Value Measurements (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Jun. 30, 2020
Level 1      
Fair Value Measurements      
Carrying value of cash held in money market fund $ 116.9 $ 135.0  
Level 2      
Fair Value Measurements      
Assets measured at fair value levels 2 or 3 0.0   $ 0.0
Liabilities measured at fair value levels 2 or 3 0.0   0.0
Level 3      
Fair Value Measurements      
Assets measured at fair value levels 2 or 3 0.0   0.0
Liabilities measured at fair value levels 2 or 3 $ 0.0   $ 0.0