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Stock-Based Compensation - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 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     $ 3,785 $ 3,819    
Dividends declared not paid on restricted stock units $ 119 $ 70 119 70    
Restricted Stock Units (RSUs)            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Fair value of restricted stock units vested and released 482 404 5,015 4,018    
Compensation cost related to stock grants and vesting of restricted stock units 2,147 1,861 3,785 3,819    
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 179 182 351 359    
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 14,433 $ 14,061 $ 14,433 $ 14,061    
Expected weighted-average period to be recognized     2 years 3 months 29 days 2 years 9 months 11 days    
Dividends declared not paid on restricted stock units $ 345   $ 345     $ 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) 0 0 10,613 16,537    
Number of shares reserved for issuance (in shares) 2,421,743 2,444,428 2,421,743 2,444,428