Loans and Allowance - Credit Quality of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 7,081,059	$ 6,751,199
Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	1,657,591	1,493,493
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	89,093	121,757
Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	714,866	612,219
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,652,782	2,562,691
Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	965,720	962,765
Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	518,699	514,021
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	92,809	86,935
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans	1,945	2,527
Other commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	387,554	394,791
Commercial Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	5,432,053	5,099,464
Commercial Pass | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	1,577,925	1,418,401
Commercial Pass | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	62,750	73,800
Commercial Pass | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	678,483	587,906
Commercial Pass | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,513,163	2,408,329
Commercial Pass | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	182,691	185,725
Commercial Pass | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	27,895	28,554
Commercial Pass | Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans	1,945	2,527
Commercial Pass | Other commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	387,201	394,222
Commercial Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	114,856	154,573
Commercial Special Mention | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	33,722	51,336
Commercial Special Mention | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	11,319	27,502
Commercial Special Mention | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	2,180	828
Commercial Special Mention | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	61,303	70,074
Commercial Special Mention | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	5,360	4,376
Commercial Special Mention | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	972	457
Commercial Special Mention | Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans
Commercial Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	150,575	129,218
Commercial Substandard | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	45,944	23,386
Commercial Substandard | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	15,024	20,018
Commercial Substandard | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	9,302	981
Commercial Substandard | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	75,765	79,769
Commercial Substandard | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	3,813	4,209
Commercial Substandard | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	374	286
Commercial Substandard | Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans
Commercial Substandard | Other commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	353	569
Commercial Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans	82	2,407
Commercial Doubtful | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans		370
Commercial Doubtful | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans		387
Commercial Doubtful | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	51	1,536
Commercial Doubtful | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	31	114
Commercial Doubtful | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans
Commercial Loss
Financing Receivable, Recorded Investment [Line Items]
Loans		50
Commercial Loss | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans		50
Commercial Loss | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans
Consumer Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	1,377,047	1,354,790
Consumer Performing | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans
Consumer Performing | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	24,836	22,374
Consumer Performing | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,499	2,980
Consumer Performing | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	768,627	759,900
Consumer Performing | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	488,329	482,661
Consumer Performing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	92,756	86,875
Consumer Non-Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	6,446	10,697
Consumer Non-Performing | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	65	130
Consumer Non-Performing | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	1	3
Consumer Non-Performing | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	5,198	8,441
Consumer Non-Performing | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,129	2,063
Consumer Non-Performing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 53	$ 60