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Stock-Based Compensation - Deferred Stock Units, DSUs (Details) - DSUs - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Units      
Balance outstanding at the beginning of the period (in shares) 331,223    
Granted (in shares) 58,861    
Converted to Common Stock (in shares) (47,378)    
Balance outstanding at the end of the period (in shares) 342,706 331,223  
Weighted Average Grant Date Fair Value per Unit      
Balance outstanding at the beginning of the period, (in usd per share) $ 23.98    
Granted (in usd per share) 35.59 $ 34.84 $ 33.43
Vested (in usd per share) 26.87    
Balance outstanding at the end of the period (in usd per share) $ 25.37 $ 23.98  
Aggregate intrinsic value for DSUs outstanding $ 13,000,000 $ 13,000,000 $ 13,000,000
Aggregate intrinsic values for DSUs converted to common stock during the period $ 2,000,000 $ 2,000,000 $ 6,000,000
Weighted average grant date fair value (in usd per share) $ 35.59 $ 34.84 $ 33.43