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Income Taxes - Schedule of Effective Income Tax Reconciliation (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]                      
Federal tax expense at statutory rate                 $ 14,090 $ 13,632 $ 13,349
Increase (reduction) in income taxes resulting from:                      
State tax, net of federal benefit                 1,253 1,275 1,098
Non-deductible compensation                     378
Tax rate change $ (3,100)               (3,122)    
Excess tax benefits from share-based awards                 (1,603)    
Research and development tax credits                 (400)    
Other                 76 10 (51)
Total $ 923 $ 3,235 $ 3,751 $ 2,385 $ 4,014 $ 4,020 $ 3,668 $ 3,215 $ 10,294 $ 14,917 $ 14,774