Loans and Allowance - Credit Quality of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 6,901,696	$ 6,751,199
Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	1,554,169	1,493,493
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	98,315	121,757
Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	590,093	612,219
Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,713,994	2,562,691
Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	948,644	962,765
Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	510,545	514,021
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	88,235	86,935
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans	2,193	2,527
Other commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	395,508	394,791
Commercial Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	5,237,859	5,099,464
Commercial Pass | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	1,465,692	1,418,401
Commercial Pass | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	57,888	73,800
Commercial Pass | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	553,131	587,906
Commercial Pass | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,557,293	2,408,329
Commercial Pass | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	179,135	185,725
Commercial Pass | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	27,372	28,554
Commercial Pass | Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans	2,193	2,527
Commercial Pass | Other commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	395,155	394,222
Commercial Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	146,964	154,573
Commercial Special Mention | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	58,916	51,336
Commercial Special Mention | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	16,599	27,502
Commercial Special Mention | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	1,363	828
Commercial Special Mention | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	64,356	70,074
Commercial Special Mention | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	5,198	4,376
Commercial Special Mention | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	532	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	159,656	129,218
Commercial Substandard | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	29,561	23,386
Commercial Substandard | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	23,828	20,018
Commercial Substandard | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	11,918	981
Commercial Substandard | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	88,312	79,769
Commercial Substandard | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	5,340	4,209
Commercial Substandard | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	344	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	1,148	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	1,117	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	1,348,581	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	23,552	22,374
Consumer Performing | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,914	2,980
Consumer Performing | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	753,126	759,900
Consumer Performing | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	480,830	482,661
Consumer Performing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	88,159	86,875
Consumer Non-Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	7,488	10,697
Consumer Non-Performing | Real estate loans - Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	129	130
Consumer Non-Performing | Real estate loans - Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2	3
Consumer Non-Performing | Real estate loans - Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	5,814	8,441
Consumer Non-Performing | Real estate loans - Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,467	2,063
Consumer Non-Performing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 76	$ 60