Loans and Leases (Loans and Leases) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	$ 42,893.3	$ 40,480.6
Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	4,773.0	4,326.3
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	21,635.6	18,487.4
Commercial | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	12,415.0	11,489.2
Commercial | Commercial and Institutional | Northern Trust Private Equity Funds
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	5,600.0	4,900.0
Commercial | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	4,773.0	4,326.3
Commercial | Non-U.S
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	3,131.1	1,990.2
Commercial | Lease Financing, net
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	0.0	11.0
Commercial | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	1,316.5	670.7
Personal
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	21,257.7	21,993.2
Personal | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	14,119.0	15,256.3
Personal | Residential Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	6,413.5	6,319.9
Personal | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	510.0	381.8
Personal | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	$ 215.2	$ 35.2