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Stock-Based Compensation - Additional Information (Details)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
shares
Mar. 31, 2017
shares
Dec. 31, 2018
USD ($)
item
shares
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Additional Employee Stock Plans Information          
Income tax expense (benefit)     $ 11,854 $ (2,409) $ 24,687
Restricted Stock Share Awards | 2010 Plan          
Additional Employee Stock Plans Information          
Compensation expenses recognized     0 0 0
Unrecognized compensation expense, net of expected forfeitures, calculated under the fair value method for shares expected to vest     0 0 0
Performance Share Unit Awards          
Additional Employee Stock Plans Information          
Compensation expenses recognized     1,880 1,768 1,382
Unrecognized compensation expense, net of expected forfeitures, calculated under the fair value method for shares expected to vest     431    
Performance Share Unit Awards | 2010 Plan          
Additional Employee Stock Plans Information          
Compensation expenses recognized     $ 2,670 1,732  
Performance period     3 years    
Number of performance shares company expects participants to earn | shares     61,700    
Number of performance shares units that will be converted into RSUs | shares 64,040 87,876      
Restricted Stock Unit Awards          
Additional Employee Stock Plans Information          
Vesting period of awards     2 years    
Restricted Stock Unit Awards | 2010 Plan          
Additional Employee Stock Plans Information          
Compensation expenses recognized         1,516
Unrecognized compensation expense, net of expected forfeitures, calculated under the fair value method for shares expected to vest     $ 839    
Minimum age of employee, attaining which awards are continued to be vested upon retirement | item     65    
Minimum service period, upon serving which awards are continued to be vested upon retirement     10 years    
Minimum age of employee along with service period condition, attaining which awards are continued to be vested upon retirement | item     55    
Maximum period following a termination of service in which the share-based award will be settled     2 months 15 days    
Percentage of RSUs issued that will immediately vest and be converted into common shares 33.33%        
Percentage of RSUs issued that will vest ratably over the remaining vesting period 66.67%        
Restricted Stock Unit Awards | 2010 Plan | Management          
Additional Employee Stock Plans Information          
Accelerated stock based compensation expense     $ 2,968 $ 619 $ 528
Restricted Stock Unit Awards | 2010 Plan | Common Stock          
Additional Employee Stock Plans Information          
Number of shares issued upon exercise of units other than options | shares     1