Other Operating Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Other Income and Expenses [Abstract]
Business Promotion	$ 81.3	$ 74.8	$ 76.7
Staff Related	46.4	35.0	34.6
FDIC Insurance Premiums	43.0	112.0	17.8
Charitable Contributions	83.8	15.6	19.4
Other Expenses	200.7	235.5	210.8
Total Other Operating Expense	455.2	472.9	359.3
Other Operating Expense [Line Items]
Charitable Contributions	83.8	$ 15.6	$ 19.4
Northern Trust Foundation
Other Income and Expenses [Abstract]
Charitable Contributions	70.0
Other Operating Expense [Line Items]
Charitable Contributions	$ 70.0