Other Operating Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Other Income and Expenses [Abstract]
Business Promotion	$ 77.2	$ 81.3	$ 74.8
Staff Related	36.1	46.4	35.0
FDIC Insurance Premiums	(17.8)	(43.0)	(112.0)
Charitable Contributions	11.8	83.8	15.6
Other Expenses	202.4	200.7	235.5
Total Other Operating Expense	345.3	455.2	472.9
Other Operating Expense [Line Items]
Charitable Contributions	11.8	83.8	15.6
FDIC Insurance Premiums	17.8	43.0	112.0
FDIC Special Assessment
Other Income and Expenses [Abstract]
FDIC Insurance Premiums		(14.7)	(84.6)
Other Operating Expense [Line Items]
FDIC Insurance Premiums		14.7	$ 84.6
FDIC Insurance Premiums release	$ 15.9
Northern Trust Foundation
Other Income and Expenses [Abstract]
Charitable Contributions		70.0
Other Operating Expense [Line Items]
Charitable Contributions		$ 70.0