Income Taxes - Reconciliation of Tax Expense at the Federal Statutory Tax Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Tax expense at the federal statutory tax rate	$ 35,424	$ 31,910	$ 28,295
Increase (decrease) in tax expense from:
Tax-exempt income, net	(663)	(571)	(621)
Modified endowment life contracts	(763)	(755)	(812)
State tax expense, net of federal tax benefit	2,898	1,469	2,303
Federal tax credits	(5,165)	(6,479)	(4,132)
Other, net	3,309	1,710	1,492
Total income taxes	$ 35,040	$ 27,284	$ 26,525