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Earnings Per Share - Summary of Numerator and Denominator of Basic and Fully Diluted Earnings Per Common Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income $ 22,022 $ 16,430 $ 47,400 $ 34,050
Less: Preferred stock dividends   1   4
Less: Income attributable to participating securities (1,231) (1,203) (2,730) (2,386)
Basic Earnings Per Share:        
Income allocated to common stockholders, Income (Numerator) $ 20,791 $ 15,228 $ 44,670 $ 31,668
Income allocated to common stockholders, Shares (Denominator) 9,580 9,925 9,560 10,092
Income allocated to common stockholders, Per Share Amount $ 2.17 $ 1.53 $ 4.67 $ 3.14
Diluted Earnings Per Share:        
Income available to common stockholders and assumed conversions, Income (Numerator) $ 21,912 $ 16,308 $ 46,901 $ 33,816
Income available to common stockholders and assumed conversions, Shares (Denominator) 11,345 11,710 11,334 11,923
Income available to common stockholders and assumed conversions, Per Share Amount $ 1.93 $ 1.39 $ 4.14 $ 2.84
Convertible Senior Notes [Member]        
Effect of Dilutive Securities:        
Dilutive Securities, Income (Numerator) $ 1,121 $ 1,081 $ 2,231 $ 2,152
Dilutive Securities, Shares (Denominator) 1,650 1,649 1,649 1,649
Convertible Preferred Stock [Member]        
Effect of Dilutive Securities:        
Dilutive Securities, Income (Numerator)   $ (1)   $ (4)
Dilutive Securities, Shares (Denominator)   3   41
Stock Options [Member]        
Effect of Dilutive Securities:        
Dilutive Securities, Shares (Denominator) 115 133 125 141