Loans and Leases - Narrative (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Loan	Dec. 31, 2017 USD ($) Loan	Dec. 31, 2016 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	$ 2,200,000,000	$ 906,400,000
Demand deposits	152,500,000	127,600,000
Loans held for sale	0	20,900,000
Leases held for sale		33,100,000
Interest income that would have been recorded for nonperforming loans	8,000,000	9,100,000	$ 8,500,000
Undrawn loan commitments and standby letters of credit	12,600,000	9,400,000
Loans and leases receivable, net of deferred income	32,490,000,000	32,592,200,000
Consumer loans collateralized by residential properties in foreclosure	10,900,000	14,100,000
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	$ 655,500,000	$ 908,600,000
Loans held by Northern trust	95.00%	93.00%
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	$ 32,380,700,000	$ 32,441,500,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	109,300,000	150,700,000
Other real estate, foreclosed assets, and repossessed assets	8,400,000	4,600,000
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	35,200,000	25,900,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	$ 64,600,000	$ 72,500,000
Redefaulted | Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans or leases modified in trouble debt restructuring | Loan	4	2