Condensed Financial Information - Parent Company - Condensed Statements of Earnings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income:
Total interest income	$ 198,539	$ 176,369	$ 159,796
Expenses:
Salaries and employee benefits	70,456	66,529	58,269
EARNINGS BEFORE INCOME TAXES	118,592	104,568	99,360
Income tax benefit	(29,033)	(25,700)	(25,135)
NET EARNINGS	89,559	78,868	74,225
Parent Company [Member]
Income:
Cash dividends from subsidiaries	34,000	64,200	58,400
Excess of earnings over dividends of subsidiaries	58,539	17,192	17,547
Other	3,717	3,118	2,682
Total interest income	96,256	84,510	78,629
Expenses:
Salaries and employee benefits	5,595	4,540	3,668
Other operating expenses	3,309	3,017	2,338
Expenses, total	8,904	7,557	6,006
EARNINGS BEFORE INCOME TAXES	87,352	76,953	72,623
Income tax benefit	2,207	1,915	1,602
NET EARNINGS	$ 89,559	$ 78,868	$ 74,225