Retirement Plans (Details) - USD ($)					12 Months Ended
	Feb. 15, 2020	Feb. 14, 2020	Jun. 14, 2019	Feb. 01, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
RAP
Retirements Plans [Line Items]
Defined Benefit Plan, Description					The Corporation has a noncontributory defined benefit retirement plan, the RAP, covering substantially all employees who meet participation requirements. The benefits are based primarily on years of service and the employee’s compensation paid. Employees of acquired entities generally participate in the RAP after consummation of the business combinations. Any retirement plans of acquired entities are typically merged into the RAP after completion of the mergers, and credit is usually given to employees for years of service at the acquired institution for vesting and eligibility purposes.
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year					$ 4,000,000
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year					$ 73,000
Actual return on plan assets percentage					15.18%	18.29%
Employer contributions					$ 0	$ 0
Postretirement Plan
Retirements Plans [Line Items]
Defined Benefit Plan, Description					The Corporation also provides legacy healthcare access to a limited group of retired employees from a previous acquisition in the Postretirement Plan. There are no other active retiree healthcare plans.
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year					$ 0
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year					75,000
Employer contributions					$ 210,000	270,000
Defined Benefit Plan Health Care Cost Rate Change					(6.00%)
Health care trend rate assumption					5.50%
Defined Benefit Plan, Health Care Cost Trend, Reduction in Trend Rate Minimum					0.25%
Ultimate future rate					5.00%
401(k) plan
Retirements Plans [Line Items]
Total expense related to 401(k)					$ 15,000,000	$ 16,000,000	$ 16,000,000
Minimum [Member] | RAP | Equity securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage					50.00%
Minimum [Member] | RAP | Fixed-income securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage					30.00%
Minimum [Member] | RAP | Other Cash Equivalents
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage					0.00%
Minimum [Member] | RAP | Alternative securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage					0.00%
Maximum [Member] | RAP | Equity securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage					70.00%
Maximum [Member] | RAP | Fixed-income securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage					50.00%
Maximum [Member] | RAP | Other Cash Equivalents
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage					10.00%
Maximum [Member] | RAP | Alternative securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage					15.00%
Bank Mutual [Member]
Retirements Plans [Line Items]
Business Acquisition, Effective Date of Acquisition				Feb. 01, 2018
Business Acquisition, Effective Date of Retirement Eligibility				Feb. 01, 2018
Huntington National Bank
Retirements Plans [Line Items]
Business Acquisition, Effective Date of Acquisition			Jun. 14, 2019
Business Acquisition, Effective Date of Retirement Eligibility			Jun. 14, 2019
First Staunton Bancshares
Retirements Plans [Line Items]
Business Acquisition, Effective Date of Acquisition		Feb. 14, 2020
Business Acquisition, Effective Date of Retirement Eligibility	Feb. 15, 2020