Other Operating Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Other Income and Expenses [Abstract]
Business Promotion	$ 20.8	$ 21.1	$ 37.5	$ 37.3
Staff Related	8.2	8.9	22.4	16.7
FDIC Insurance Premiums	9.8	6.6	28.5	12.8
Charitable Contributions	74.0	4.3	76.0	7.5
Other Expenses	62.5	71.1	111.3	123.3
Total Other Operating Expense	175.3	112.0	275.7	197.6
FDIC, special assessment	2.3		14.7
Other Operating Expense [Line Items]
Charitable Contributions	74.0	$ 4.3	76.0	$ 7.5
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