Condensed Financial Information - Parent Company - Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income:
Total interest income	$ 221,623	$ 198,539	$ 176,369
Expenses:
Salaries and employee benefits	80,999	70,456	66,529
EARNINGS BEFORE INCOME TAXES	131,818	118,592	104,568
Income tax benefit	(31,437)	(29,033)	(25,700)
NET EARNINGS	100,381	89,559	78,868
Parent Company [Member]
Income:
Cash dividends from subsidiaries	51,200	34,000	64,200
Excess of earnings over dividends of subsidiaries	52,911	58,539	17,192
Other	4,185	3,717	3,118
Total interest income	108,296	96,256	84,510
Expenses:
Salaries and employee benefits	6,067	5,595	4,540
Other operating expenses	4,439	3,309	3,017
Total Expense	10,506	8,904	7,557
EARNINGS BEFORE INCOME TAXES	97,790	87,352	76,953
Income tax benefit	2,591	2,207	1,915
NET EARNINGS	$ 100,381	$ 89,559	$ 78,868