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Share-based Compensation - Directors' Stock Grants (Details) - USD ($)
shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award      
Nonemployee directors stock awards issued (shares)   10 20
Nonemployee directors stock awards, Amount recognized in equity, fair value   $ 0.7 $ 0.7
Nonemployee      
Share-based Compensation Arrangement by Share-based Payment Award      
Directors stock awards issued (shares) (shares) 10    
Fair value of shares vested $ 0.6