Quarterly report pursuant to Section 13 or 15(d)

Employee Stock Plans - Additional Information (Details)

v3.10.0.1
Employee Stock Plans - Additional Information (Details)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
age
shares
Jun. 30, 2017
USD ($)
Additional Employee Stock Plans Information        
Income tax expense $ 6,916 $ 7,608 $ 5,654 $ 4,914
Performance Share Unit Awards | 2010 Plan        
Additional Employee Stock Plans Information        
Vesting period of awards     2 years  
Compensation expenses recognized 1,442 520 $ 1,932 671
Unrecognized compensation expense, net of expected forfeitures, calculated under the fair value method for shares expected to vest 601   601  
Restricted Stock Unit Awards        
Additional Employee Stock Plans Information        
Compensation expenses recognized 1,303 238 2,233 1,437
Unrecognized compensation expense, net of expected forfeitures, calculated under the fair value method for shares expected to vest 1,266   $ 1,266  
Restricted Stock Unit Awards | Non-employee director        
Additional Employee Stock Plans Information        
Number of shares issued upon exercise of units other than options | shares     1  
Restricted Stock Unit Awards | Management        
Additional Employee Stock Plans Information        
Minimum age of employee, attaining which awards are continued to be vested upon retirement | age     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 | age     55  
Accelerated stock based compensation expense $ 2,226 $ 0 $ 2,968 $ 619
Restricted Stock Unit Awards | Common Stock | Management        
Additional Employee Stock Plans Information        
Number of shares issued upon exercise of units other than options | shares     1  
Restricted Stock Unit Awards | 2010 Plan        
Additional Employee Stock Plans Information        
Maximum period following a termination of service in which the share-based award will be settled     2 months 15 days