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Income Taxes - Current and Deferred Tax Expense (Benefit) (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
Current:                      
Federal                 $ 4,163 $ 32,506 $ 33,778
State                 1,731 1,314 2,337
Total current                 5,894 33,820 36,115
Deferred:                      
Federal                 39,003 2,606 (525)
State                 1,631 554 851
Total deferred                 40,634 3,160 326
Income tax expense (benefit) $ 15,740 $ 11,969 $ 10,451 $ 8,368 $ 10,182 $ 9,281 $ 8,781 $ 8,736 $ 46,528 $ 36,980 $ 36,441