Loans and Leases - Narrative (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Loan	Dec. 31, 2016 USD ($) Loan	Dec. 31, 2015 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Lower limit of generally required loan to collateral value for residential real estate loans	65.00%
Upper limit of generally required loan to collateral value for residential real estate loans	80.00%
Draw period for credit line products	10 years
Other U.S. loans and non-U.S. loans, short duration advances	$ 906.4	$ 1,400.0
Demand deposits	127.6	88.1
Loans held for sale	20.9	13.4
Leases held for sale	33.1	43.0
Interest income that would have been recorded for nonperforming loans	9.1	8.5	$ 8.1
Undrawn loan commitments and standby letters of credit	9.4	2.3
Loans and leases receivable, net of deferred income	32,592.2	33,822.1
Foreclosed residential real estate properties, carrying value	4.3	4.6
Consumer loans collateralized by residential properties in foreclosure	14.1	25.9
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	$ 908.6	$ 1,200.0
Loans held by Northern trust	93.00%	91.00%
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	$ 32,441.5	$ 33,661.9
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	150.7	160.2
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	25.9	42.4
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	$ 72.5	$ 85.2
Redefaulted | Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans or leases modified in trouble debt restructuring | Loan	2	5