Retirement Plans - Funded Status of the Pension Plans and Postretirement Benefit Plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amounts recognized in the Consolidated Statements of Financial Condition:
Other noninterest expense	$ 332,751,000	$ 303,716,000	$ 255,486,000
Pension Plan
Change in benefit obligation:
Benefit obligation, beginning of period	165,976,000	28,330,000
Benefit obligation acquired in TCF/Chemical Merger	0	136,587,000
Service cost	0	0	0
Interest cost	3,785,000	3,013,000	983,000
Net actuarial (gain) loss	(782,000)	(3,831,000)
Plan amendment	0	0
Benefits paid	(170,543,000)	(5,785,000)
Benefit obligation, end of period	0	165,976,000	28,330,000
Change in fair value of plan assets:
Fair value of plan assets, beginning of period	169,484,000	32,844,000
Fair value of plan assets acquired in TCF/Chemical Merger	0	141,746,000
Actual gain on plan assets	12,551,000	333,000
Benefits paid	(170,543,000)	(5,439,000)
Employer contributions	2,423,000	0	0
Fair value of plan assets, end of period	13,915,000	169,484,000	32,844,000
Funded status of plan, end of period	13,915,000	3,508,000
Benefit obligation:
Accumulated benefit obligation	0	165,976,000
Amounts recognized in the Consolidated Statements of Financial Condition:
Prepaid (accrued) benefit cost, end of period		3,508,000
Prior service cost included in accumulated other comprehensive income (loss)	0	0
Pension Plan | Prepaid Expenses and Other Current Assets
Amounts recognized in the Consolidated Statements of Financial Condition:
Prepaid (accrued) benefit cost, end of period	0
Pension Plan | Other Expense
Amounts recognized in the Consolidated Statements of Financial Condition:
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	8,500,000
Postretirement Plan
Change in benefit obligation:
Benefit obligation, beginning of period	5,276,000	3,320,000
Benefit obligation acquired in TCF/Chemical Merger	0	2,271,000
Service cost	1,000	1,000	0
Interest cost	143,000	149,000	110,000
Net actuarial (gain) loss	(295,000)	19,000
Plan amendment	(449,000)	0	0
Benefits paid	(460,000)	(446,000)
Benefit obligation, end of period	4,806,000	5,276,000	3,320,000
Change in fair value of plan assets:
Fair value of plan assets, beginning of period	0	0
Fair value of plan assets acquired in TCF/Chemical Merger	0	0
Actual gain on plan assets	0	0
Benefits paid	(460,000)	(377,000)
Employer contributions	460,000	377,000	392,000
Fair value of plan assets, end of period	0	0	$ 0
Funded status of plan, end of period	(4,806,000)	(5,276,000)
Benefit obligation:
Accumulated benefit obligation	0	0
Amounts recognized in the Consolidated Statements of Financial Condition:
Prepaid (accrued) benefit cost, end of period	(4,806,000)	(5,276,000)
Prior service cost included in accumulated other comprehensive income (loss)	(504,000)	$ (101,000)
Postretirement Plan | Legacy TCF
Change in fair value of plan assets:
Employer contributions	$ 264,000