Loans and Leases - Narrative (Detail)	12 Months Ended			13 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) Loan	Dec. 31, 2017 USD ($)	Jun. 30, 2019 USD ($) Loan	Mar. 31, 2018 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	$ 1,100,000,000	$ 2,200,000,000
Demand deposits	90,400,000	152,500,000
Interest income that would have been recorded for nonperforming loans	7,300,000	8,000,000.0	$ 9,100,000
Undrawn loan commitments and standby letters of credit	9,100,000	12,600,000
Loans and leases receivable, net of deferred income	31,409,600,000	32,490,000,000.0
Financing receivable, troubled debt restructuring	6,100,000		$ 2,100,000	$ 5,800,000	$ 2,400,000
Consumer loans collateralized by residential properties in foreclosure	18,100,000	10,900,000
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	$ 448,500,000	$ 655,500,000
Loans held by Northern trust	97.00%	95.00%
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	$ 31,326,000,000.0	$ 32,380,700,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	83,600,000	109,300,000
Other real estate, foreclosed assets, and repossessed assets	3,200,000	8,400,000
Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	0	0
Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for sale	53,600,000	0
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	27,700,000	35,200,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases receivable, net of deferred income	$ 54,900,000	$ 64,600,000
Redefaulted | Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans or leases modified in trouble debt restructuring | Loan		4		5