Other Operating Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Other Income and Expenses [Abstract]
Business Promotion	$ 17.1	$ 19.1	$ 56.2	$ 56.6
Staff Related	13.9	12.4	25.1	34.8
FDIC Insurance Premiums	2.2	7.1	19.6	35.6
Charitable Contributions	3.4	3.3	8.4	79.3
Other Expenses	48.4	44.2	150.6	155.5
Total Other Operating Expense	85.0	86.1	259.9	361.8
Other Operating Expense [Line Items]
Charitable contributions	$ 3.4	3.3	$ 8.4	79.3
Northern Trust Foundation
Other Income and Expenses [Abstract]
Charitable Contributions		70.0		70.0
Other Operating Expense [Line Items]
Charitable contributions		$ 70.0		70.0
FDIC Special Assessment
Other Operating Expense [Line Items]
FDIC special assessment charge				$ 14.7