Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Line Items]
Statutory federal tax rate	35.00%
Unrecognized tax benefit	$ 11,900,000	$ 15,600,000	$ 19,400,000
Increase in net income from recognition of unrecognized tax benefits	8,800,000	12,600,000
Net (decreases) increases in unrecognized tax benefits	(3,700,000)	(3,800,000)
Interest and penalties, net of tax, included in the provision (benefit) for income taxes	200,000	(1,700,000)
Liability for the potential payment of interest and penalties	10,200,000	11,000,000
Earnings of certain non-U.S. subsidiaries indefinitely reinvested	177,400,000	141,000,000	137,400,000
Approximated cumulative amount of undistributed pre-tax earnings in subsidiaries	1,100,000,000
Additional deferred tax liability if Northern Trust had not elected to indefinitely reinvest foreign earnings	255,000,000
Net operating loss carryforwards	3,900,000
Valuation allowance related to net operating loss carryforwards	3,900,000	3,900,000	3,900,000
Valuation Reserve	3,900,000	3,900,000	3,900,000
Remaining Deferred Tax Assets
Income Taxes [Line Items]
Valuation Reserve	0	0	0
Leveraged Lease Obligations | Income Tax Expense Benefit
Income Taxes [Line Items]
Application of new tax estimates			$ (12,400,000)