Condensed Financial Information - Parent Company - Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income:
Total interest income	$ 232,288	$ 221,623	$ 198,539
Expenses:
Salaries and employee benefits	90,739	80,999	70,456
EARNINGS BEFORE INCOME TAXES	135,927	131,818	118,592
Income tax benefit	(31,153)	(31,437)	(29,033)
NET EARNINGS	104,774	100,381	89,559
Parent Company [Member]
Income:
Cash dividends from subsidiaries	48,800	51,200	34,000
Excess of earnings over dividends of subsidiaries	58,809	52,911	58,539
Other	4,184	4,185	3,717
Total interest income	111,793	108,296	96,256
Expenses:
Salaries and employee benefits	5,655	6,067	5,595
Other operating expenses	3,531	4,439	3,309
Total Expense	9,186	10,506	8,904
EARNINGS BEFORE INCOME TAXES	102,607	97,790	87,352
Income tax benefit	2,167	2,591	2,207
NET EARNINGS	$ 104,774	$ 100,381	$ 89,559