Annual report pursuant to Section 13 and 15(d)

Stock Based Compensation - Additional Information (Details)

v3.8.0.1
Stock Based Compensation - Additional Information (Details)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
shares
Mar. 31, 2017
shares
Mar. 31, 2016
shares
Dec. 31, 2017
USD ($)
item
shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Additional Employee Stock Plans Information            
Income tax expense       $ (2,409) $ 24,687 $ 22,087
Restricted Stock Share Awards | 2010 Plan            
Additional Employee Stock Plans Information            
Compensation expenses recognized       0 0 385
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       1,768 1,382 1,247
Unrecognized compensation expense, net of expected forfeitures, calculated under the fair value method for shares expected to vest       424    
Performance Share Unit Awards | 2010 Plan            
Additional Employee Stock Plans Information            
Compensation expenses recognized       $ 1,732 1,516  
Number of performance shares units that will be converted into RSUs | shares 64,040 87,876 71,428      
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           1,643
Unrecognized compensation expense, net of expected forfeitures, calculated under the fair value method for shares expected to vest       $ 461    
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       $ 619 $ 528 $ 303
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    
Accounting Standards Update 2016-09            
Additional Employee Stock Plans Information            
Income tax expense       $ 616