Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Tax Credit Carryforward [Line Items]
Statutory federal tax rate	35.00%
Unrecognized tax benefit	$ 17,200,000	$ 12,300,000	$ 11,900,000
Increase in net income from recognition of unrecognized tax benefits	11,900,000	8,700,000
Increases in unrecognized tax benefits	4,900,000	400,000
Interest and penalties, net of tax, included in the provision (benefit) for income taxes	1,600,000	100,000
Liability for the potential payment of interest and penalties	9,900,000	11,300,000
Earnings of certain non-U.S. subsidiaries indefinitely reinvested	237,100,000	257,400,000	177,400,000
Approximated cumulative amount of undistributed pre-tax earnings in subsidiaries	1,600,000,000
Additional deferred tax liability if Northern Trust had not elected to indefinitely reinvest foreign earnings	356,000,000
Valuation allowance	900,000	1,600,000	3,900,000
State
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards	900,000	1,600,000	3,900,000
Valuation allowance	$ 0	$ 0	$ 0