Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest and Dividend Income, Operating [Abstract]
Interest and Fee Income, Loans and Leases			$ 328,727	$ 321,385	$ 271,145
Interest and Dividend Income, Securities, by Classification [Abstract]
Interest Income, Securities, US Treasury and Other US Government			19,818	26,213	29,479
Interest Income, Securities, State and Municipal			8,436	8,955	8,770
Other Interest and Dividend Income			739	3,947	1,407
Interest and Dividend Income, Operating, Total	$ 91,800	$ 88,909	357,720	360,500	310,801
Interest Expense, Deposits [Abstract]
Interest Expense, NOW Accounts, Money Market Accounts, and Savings Deposits			9,142	33,348	19,815
Interest Expense, Time Deposits			12,186	17,494	12,375
Interest Expense, Borrowings [Abstract]
Interest Expense, Short-term Borrowings			1,110	2,476	1,600
Interest Expense, Long-term Debt			7,652	9,445	10,113
Total interest expense	5,479	12,626	30,090	62,763	43,903
Net interest income	86,321	76,283	327,630	297,737	266,898
Provision for loan losses	(19,159)	5,153	12,054	6,171	7,945
Interest Income (Expense), after Provision for Loan Loss, Total			315,576	291,566	258,953
Noninterest Income [Abstract]
Debit card fee income			22,160	20,250	17,317
Bank Owned Life Insurance Income			4,789	4,557	6,815
ATM Fees			1,773	1,828	1,978
Gains (Losses) on Sales of Other Real Estate			1,207	(222)	4,235
Gain (Loss) on Sale of Investments			3,286	(421)	(2,271)
Gain on equity securities, net			2,182	5,118	4,616
Net Periodic Defined Benefits Expense (Reversal of Expense), Excluding Service Cost Component			7,952	4,732	6,820
Gain (Loss) on Sales of Loans, Net			0	0	2,826
Noninterest Income, Other			7,386	7,248	6,745
Total other income			125,664	97,193	101,101
Noninterest Expense [Abstract]
Salaries			128,040	119,514	103,755
Employee benefits			37,115	36,806	30,289
Occupancy expense			13,802	12,815	11,251
Furniture and equipment expense			18,805	17,032	16,139
Data processing fees			11,659	10,750	8,477
Professional fees and services			31,303	33,317	28,894
Marketing			5,828	5,753	5,144
Insurance			6,423	3,130	5,289
Communication			4,084	5,351	4,981
State tax expense			3,991	3,829	3,813
Amortization of Intangible Assets			2,263	2,355	578
Payment for Debt Extinguishment or Debt Prepayment Cost	8,700	1,800	10,529	612	197
Foundation contribution expense			3,000	1,500	3,000
Miscellaneous			9,753	11,224	6,948
Total other expense			286,595	263,988	228,755
Income before income taxes	55,475	27,340	154,645	124,771	131,299
Income taxes			26,722	22,071	20,912
Net Income (Loss) Attributable to Parent	$ 45,200	$ 22,372	$ 127,923	$ 102,700	$ 110,387
Earnings Per Share [Abstract]
Basic (in dollars per share)	$ 2.77	$ 1.37	$ 7.85	$ 6.33	$ 7.13
Diluted (in dollars per share)	$ 2.75	$ 1.36	$ 7.80	$ 6.29	$ 7.07
Fiduciary and Trust [Member]
Noninterest Income [Abstract]
Revenue from Contract with Customer, Including Assessed Tax			$ 28,873	$ 27,768	$ 26,293
Deposit Account [Member]
Noninterest Income [Abstract]
Revenue from Contract with Customer, Including Assessed Tax			8,445	10,835	11,461
Bank Servicing [Member]
Noninterest Income [Abstract]
Revenue from Contract with Customer, Including Assessed Tax			37,611	15,500	14,266
Noninterest Income, Other			$ 37,600	$ 15,500	$ 14,300