Consolidated Statements of Earnings - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
INTEREST INCOME:
Interest and fees on loans	$ 200,347	$ 166,807	$ 161,018
Interest on investment securities:
Taxable	50,052	32,825	27,626
Exempt from federal income tax	39,661	44,659	43,302
Interest on federal funds sold and interest-bearing deposits in banks	1,630	1,684	342
Total interest income	291,690	245,975	232,288
INTEREST EXPENSE:
Interest on deposits	16,946	8,213	4,503
Other	1,984	1,075	948
Total interest expense	18,930	9,288	5,451
Net interest income	272,760	236,687	226,837
PROVISION FOR LOAN LOSSES	5,665	6,530	10,212
Net interest income after provision for loan losses	267,095	230,157	216,625
NONINTEREST INCOME:
ATM, interchange and credit card fees	28,532	25,686	23,910
Net gain on sale of available-for-sale securities (includes $1,354, $1,828 and $1,270 for the years ended December 31, 2018, 2017 and 2016, respectively, related to accumulated comprehensive earnings reclassifications)	1,354	1,828	1,270
Net gain (loss) on sale of foreclosed assets	116	(50)	456
Net gain (loss) on sale of assets	(147)	(396)	168
Interest on loan recoveries	938	1,128	2,112
Other	5,970	4,602	3,108
Total noninterest income	101,764	91,017	85,132
NONINTEREST EXPENSE:
Salaries and employee benefits	105,189	95,287	90,739
Loss from partial settlement of pension plan	1,546		267
Net occupancy expense	11,173	10,521	10,420
Equipment expense	13,841	13,765	13,479
FDIC insurance premiums	2,333	2,217	2,680
ATM, interchange and credit card expenses	9,282	7,452	7,231
Professional and service fees	8,894	8,063	6,877
Printing, stationery and supplies	1,997	1,989	2,093
Operational and other losses	2,188	3,192	2,170
Amortization of intangible assets	1,272	613	738
Other	32,969	30,887	29,136
Total noninterest expense	190,684	173,986	165,830
EARNINGS BEFORE INCOME TAXES	178,175	147,188	135,927
INCOME TAX EXPENSE (includes $284, $640 and $445 for the years ended December 31, 2018, 2017 and 2016, respectively, related to income tax expense from reclassification items)	27,537	26,817	31,153
NET EARNINGS	$ 150,638	$ 120,371	$ 104,774
NET EARNINGS PER SHARE, BASIC	$ 2.23	$ 1.82	$ 1.59
NET EARNINGS PER SHARE, ASSUMING DILUTION	$ 2.22	$ 1.81	$ 1.59
Trust Fees [Member]
NONINTEREST INCOME:
Revenue from contract with customer	$ 28,181	$ 23,694	$ 19,636
Service Charges on Deposit Accounts [Member]
NONINTEREST INCOME:
Revenue from contract with customer	21,663	19,416	18,386
Real Estate Mortgage Operations [Member]
NONINTEREST INCOME:
Revenue from contract with customer	$ 15,157	$ 15,109	$ 16,086