Condensed Financial Information - Parent Company - Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income:
Total interest income	$ 291,690	$ 245,975	$ 232,288
Expenses:
Salaries and employee benefits	105,189	95,287	90,739
EARNINGS BEFORE INCOME TAXES	178,175	147,188	135,927
Income tax benefit	(27,537)	(26,817)	(31,153)
Net earnings	150,638	120,371	104,774
Parent Company [Member]
Income:
Cash dividends from subsidiaries	74,100	30,800	48,800
Excess of earnings over dividends of subsidiaries	82,323	92,929	58,809
Other	7,269	6,590	4,184
Total interest income	163,692	130,319	111,793
Expenses:
Salaries and employee benefits	9,966	8,606	5,655
Other operating expenses	4,781	3,871	3,531
Total Expense	14,747	12,477	9,186
EARNINGS BEFORE INCOME TAXES	148,945	117,842	102,607
Income tax benefit	1,693	2,529	2,167
Net earnings	$ 150,638	$ 120,371	$ 104,774