Loans (Loans and Leases) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	$ 47,617.0	$ 42,893.3
Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	5,134.2	4,773.0
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	25,412.8	21,635.6
Commercial | Commercial and Institutional
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	11,555.3	12,415.0
Commercial | Commercial and Institutional | Northern Trust Private Equity Funds
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	4,500.0	5,600.0
Commercial | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	5,134.2	4,773.0
Commercial | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	2,778.5	3,131.1
Commercial | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	5,944.8	1,316.5
Personal
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	22,204.2	21,257.7
Personal | Private Client
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	14,360.0	14,119.0
Personal | Residential Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	6,327.1	6,413.5
Personal | Non-U.S.
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	428.8	510.0
Personal | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total loans and leases	$ 1,088.3	$ 215.2