Retirement Plans (Details) - USD ($)			12 Months Ended
	Jun. 14, 2019	Feb. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
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			$ 3,000,000
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year			$ 75,000
Actual return on plan assets percentage			18.29%	(3.69%)
Employer contributions			$ 0	$ 4,340,000
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			$ 270,000	292,000
Postretirement Plan | Post-65 Health Care Trend [Member]
Retirements Plans [Line Items]
Defined Benefit Plan Health Care Cost Rate Change			2.00%
Health care trend rate assumption			5.75%
Defined Benefit Plan, Health Care Cost Trend, Reduction in Trend Rate Minimum			0.25%
Postretirement Plan | Pre-65 Health Care Trend Rate [Member]
Retirements Plans [Line Items]
Ultimate future rate			5.00%
Bank Mutual Pension
Retirements Plans [Line Items]
Employer contributions				37,537,000
401(k) plan
Retirements Plans [Line Items]
Total expense related to 401(k)			$ 16,000,000	$ 16,000,000	$ 14,000,000
Minimum [Member] | Pension Plan [Member] | Equity securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			50.00%
Minimum [Member] | Pension Plan [Member] | Fixed-income securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			30.00%
Minimum [Member] | Pension Plan [Member] | Other Cash Equivalents
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			0.00%
Minimum [Member] | Pension Plan [Member] | Alternative securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			0.00%
Maximum [Member] | Pension Plan [Member] | Equity securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			70.00%
Maximum [Member] | Pension Plan [Member] | Fixed-income securities
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			50.00%
Maximum [Member] | Pension Plan [Member] | Other Cash Equivalents
Retirements Plans [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage			10.00%
Maximum [Member] | Pension Plan [Member] | 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
Huntington National Bank
Retirements Plans [Line Items]
Business Acquisition, Effective Date of Acquisition	Jun. 14, 2019