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Earnings per Common Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Earnings Per Common Share [Line Items]                      
Conversion price (in dollars per share) $ 101.38               $ 101.38    
Numerator — basic and diluted                      
Net income $ 23,724 $ 44,333 $ 43,609 $ 38,945 $ 66,888 $ 32,214 $ (5,156) $ 14,016 $ 150,611 $ 107,962 $ 85,520
Denominator                      
Weighted average number of common shares outstanding — basic (in shares) 37,368 37,318 37,001 36,700 36,906 37,431 39,555 40,527 37,098 38,697 40,943
Effect of dilutive stock options (in shares)                 491 117 281
Effect of dilutive restricted shares (in shares)                 729 378 485
Weighted average number of common shares outstanding — diluted (in shares) 38,628 38,756 38,271 37,612 37,643 37,746 39,555 41,245 38,318 39,192 41,709
Earnings per common share — basic (in dollars per share) $ 0.63 $ 1.19 $ 1.18 $ 1.06 $ 1.81 $ 0.86 $ (0.13) $ 0.35 $ 4.06 $ 2.79 $ 2.09
Earnings per common share — diluted (in dollars per share) $ 0.61 $ 1.14 $ 1.14 $ 1.04 $ 1.78 $ 0.85 $ (0.13) $ 0.34 $ 3.93 $ 2.75 $ 2.05
Stock Options And Restricted Shares                      
Denominator                      
Antidilutive stock options and restricted shares (in shares)                 175 1,561 1,404