Income Taxes - Schedule of Effective Income Tax Reconciliation (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal tax expense at statutory rate	21.00%	35.00%	21.00%	35.00%	35.00%
Increase (reduction) in income taxes resulting from:
State tax, net of federal benefit	3.20%	3.50%	3.30%	2.90%
Excess tax benefits from share-based awards	0.00%	(0.40%)	(1.00%)	(5.40%)
Other	(1.10%)	(0.20%)	0.20%	0.40%
Effective tax rate	23.10%	37.90%	23.50%	32.90%
Federal tax expense at statutory rate	$ 2,034	$ 2,990	$ 6,743	$ 9,957
Increase (reduction) in income taxes resulting from:
State tax, net of federal benefit	306	297	1,074	821
Excess tax benefits from share-based awards		(37)	(308)	(1,525)
Other	(103)	(15)	26	118
Total	$ 2,237	$ 3,235	$ 7,535	$ 9,371