Loans and Leases - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Contract	Dec. 31, 2014 USD ($) Contract	Dec. 31, 2013 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	75.00%
Draw period for credit line products	10 years
Other U.S. loans and non-U.S. loans, short duration advances	$ 719.5	$ 1,500.0
Demand deposits	75.4	92.1
Loans held for sale	12.0	2.5
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group, Other	112.3
Interest income that would have been recorded for nonperforming loans	8.1	9.1	$ 10.6
Undrawn loan commitments and standby letters of credit	3.1	2.4
Total Loans and Leases	$ 33,180.9	$ 31,640.2
Loans or leases modified in trouble debt restructuring | Contract	10	0
Foreclosed residential real estate properties, carrying value	$ 7.9
Consumer loans collateralized by residential properties in foreclosure	$ 24.5
First Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held by Northern trust	89.00%
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	$ 1,500.0	$ 1,800.0
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	37.9	68.6
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	$ 79.2	$ 82.7