Income Taxes - Reconciliation of Tax Expense at the Federal Statutory Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Tax expense at the federal statutory tax rate	$ 37,778	$ 35,424	$ 31,910
Increase (decrease) in tax expense from:
Tax-exempt income, net	(756)	(663)	(571)
Modified endowment life contracts	(852)	(763)	(755)
State tax expense, net of federal tax benefit	3,250	2,898	1,469
Utilization of tax credits:
New markets tax credits	(1,254)	(1,195)	(1,108)
Low-income housing tax credits, net of amortization	(1,424)	(1,274)	(1,209)
Other tax credits	(319)	(248)	(1,979)
Other, net	840	861	(473)
Total income taxes	$ 37,263	$ 35,040	$ 27,284