Other Operating Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Other Income and Expenses [Abstract]
Business Promotion	$ 19.1	$ 16.2	$ 56.6	$ 53.5
Staff Related	12.4	11.0	34.8	27.7
FDIC Insurance Premiums	7.1	8.3	35.6	21.1
Charitable Contributions	3.3	3.8	79.3	11.3
Other Expenses	44.2	59.2	155.5	182.5
Total Other Operating Expense	86.1	98.5	361.8	296.1
FDIC, special assessment			14.7
Other Operating Expense [Line Items]
Charitable Contributions	$ 3.3	$ 3.8	79.3	$ 11.3
Northern Trust Foundation
Other Income and Expenses [Abstract]
Charitable Contributions			70.0
Other Operating Expense [Line Items]
Charitable Contributions			$ 70.0