|**Net Income Per Share (Tables)**                            | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|**Earnings Per Share [Abstract]**                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Reconciliation of Basic and Diluted Net Income Per Share     | The following table reconciles basic and diluted net income per share for the years indicated:201820172016Weighted-Average SharesWeighted-Average SharesWeighted-Average SharesBasic net income per share:Net income available to common stockholders$159,13949,262,015$3.23$96,07045,181,221$2.13$81,05140,767,343$1.99Effect of dilutive stock options and restricted stock awards208,908221,757245,790Diluted net income per share:Net income available to common stockholders$159,13949,470,923$3.22$96,07045,402,978$2.12$81,05141,013,133$1.98
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|                                                             | 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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|Basic net income per share:                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income available to common stockholders                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Effect of dilutive stock options and restricted stock awards |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Diluted net income per share:                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Net income available to common stockholders                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

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duration X- DefinitionTabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations.
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