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Stock-Based Compensation - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2016
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Compensation cost related to stock grants and vesting of restricted stock units     $ 5,621 $ 5,327    
Dividends declared not paid on restricted stock units $ 168 $ 134 168 134    
Restricted Stock Units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Fair value of restricted stock units vested and released 1,488 490 6,503 4,508    
Compensation cost related to stock grants and vesting of restricted stock units 1,836 1,508 5,621 5,327    
One-time expense for modification of vesting terms       249    
Restricted Stock Units (RSUs) | Director            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Compensation cost related to stock grants and vesting of restricted stock units 186 212 537 557    
Stock-Settled EBI Units and Restricted Stock Units            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Unrecognized compensation cost related to nonvested stock-settled EBI units and restricted stock units 12,759 $ 12,768 $ 12,759 $ 12,768    
Expected weighted-average period to be recognized     1 year 10 months 13 days 2 years 3 months 29 days    
Dividends declared not paid on restricted stock units $ 394   $ 394     $ 226
Employee Stock Purchase Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Purchase price percentage of subsequent offering periods         95.00%  
Maximum number of shares issuable (in shares)         200,000  
Maximum number of shares per participant (in shares)         725  
Maximum worth of award per participant         $ 25,000  
Shares issued under plan (in shares) 12,558,000 12,072,000 23,171,000 28,609,000    
Number of shares reserved for issuance (in shares) 2,409,185 2,432,356 2,409,185 2,432,356