Income Taxes - Components of the Company's Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current taxes:
Federal	$ 39,569	$ 34,003	$ 32,348
State	5,914	5,308	4,595
Deferred taxes	4,383	(2,048)	(1,903)
Total income taxes	$ 49,866	$ 37,263	$ 35,040