Condensed Financial Information - Parent Company - Condensed Statements of Earnings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income:
Income, total	$ 159,796	$ 160,021	$ 149,699
Expenses:
Salaries and employee benefits	58,269	56,256	52,641
EARNINGS BEFORE INCOME TAXES AND EXTRAORDINARY ITEM	99,360	92,185	78,431
Income tax benefit	(25,135)	(23,816)	(20,068)
Net earnings	74,225	68,369	59,659
Parent company [Member]
Income:
Cash dividends from subsidiaries	58,400	47,350	41,050
Excess of earnings over dividends of subsidiary bank	17,547	22,722	20,149
Other income	2,682	2,435	1,991
Income, total	78,629	72,507	63,190
Expenses:
Salaries and employee benefits	3,668	3,493	2,833
Other operating expenses	2,338	2,199	2,068
Expenses, total	6,006	5,692	4,901
EARNINGS BEFORE INCOME TAXES AND EXTRAORDINARY ITEM	72,623	66,815	58,289
Income tax benefit	1,602	1,554	1,370
Net earnings	$ 74,225	$ 68,369	$ 59,659