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Income Taxes - Reconciliation of Tax Expense at the Federal Statutory Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax expense at the federal statutory tax rate	$ 47,708	$ 37,778	$ 35,424
Increase (decrease) in tax expense from:
Tax-exempt income, net	(829)	(756)	(663)
Modified endowment life contracts	(921)	(852)	(763)
Share based compensation excess tax benefit	(2,354)
State tax expense, net of federal tax benefit	3,840	3,250	2,898
Write-down of net deferred tax asset	4,331
Utilization of tax credits:
New markets tax credits	(1,151)	(1,254)	(1,195)
Low-income housing tax credits, net of amortization	(1,589)	(1,424)	(1,274)
Other tax credits		(319)	(248)
Other, net	831	840	861
Total income taxes	$ 49,866	$ 37,263	$ 35,040

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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