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Income Tax - Reconciliation of U.S. Statutory Income Tax Rate to Effective Income Tax Rate (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Federal income tax expense at statutory rate $ 56,570 $ 45,257 $ 37,949
State income tax, net of federal income tax benefit 4,824 4,807 3,715
Domestic production activities deduction (2,617) (3,947) (466)
Excess tax benefits related to stock option exercises (40,624) (29,582) 0
Tax Act adjustments (21,625) 0 0
Tax credits (3,578) 0 0
Non-deductible business expenses 4,573 2,979 2,414
Other, net 160 (64) (57)
Income tax expense benefit $ (2,317) $ 19,450 $ 43,555