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Equity - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2015
Feb. 28, 2016
Sep. 30, 2015
Nov. 30, 2014
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2014
Disclosure of equity [line items]                      
Dividend payable annual percentage   40.00%                  
Dividends payable per share         $ 0.75 $ 0.75 $ 0.51 $ 0.51   $ 0.51  
Treasury stock [member]                      
Disclosure of equity [line items]                      
Share repurchase program authorized amount       $ 250,000,000              
Number of shares repurchase program                 167,650    
Share repurchase program period value                 $ 17,900,000    
Accelerated share repurchase [member] | Citibank one [member]                      
Disclosure of equity [line items]                      
Number of shares repurchase program 1,960,250                    
Share repurchase program period value     $ 100,000,000                
Stock repurchase program maturity period     3 months                
Share repurchase program settlement amount per share $ 51.01                    
Class A common stock [member]                      
Disclosure of equity [line items]                      
Common stock, shares authorised         80,000,000            
Common stock, shares issued         33,776,480         33,743,286  
Common stock, shares outstanding 31,017,102       31,185,641         31,112,356 33,050,298
Class B common stock [member]                      
Disclosure of equity [line items]                      
Common stock, shares authorised         80,000,000            
Common stock, shares issued         10,938,125         10,938,125  
Common stock, shares outstanding 10,938,125       10,938,125         10,938,125 10,938,125
Class C common stock [member]                      
Disclosure of equity [line items]                      
Common stock, shares authorised         80,000,000            
Common stock, shares outstanding         0         0