Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Tax Credit Carryforward [Line Items]
Statutory federal tax rate	35.00%
Unrecognized tax benefit	$ 12,300,000	$ 11,900,000	$ 15,600,000
Increase in net income from recognition of unrecognized tax benefits	8,700,000	8,800,000
Net (decreases) increases in unrecognized tax benefits	400,000	(3,700,000)
Interest and penalties, net of tax, included in the provision (benefit) for income taxes	100,000	200,000
Liability for the potential payment of interest and penalties	11,300,000	10,200,000
Earnings of certain non-U.S. subsidiaries indefinitely reinvested	257,400,000	177,400,000	141,000,000
Approximated cumulative amount of undistributed pre-tax earnings in subsidiaries	1,400,000,000
Additional deferred tax liability if Northern Trust had not elected to indefinitely reinvest foreign earnings	298,700,000
Valuation allowance	1,600,000	3,900,000	3,900,000
State
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards	1,600,000	3,900,000	3,900,000
Valuation allowance related to net operating loss carryforwards	1,600,000	3,900,000	3,900,000
Valuation allowance	$ 0	$ 0	$ 0