Net Periodic Pension Expense of Northern Trust's U.S. and Non-U.S. Pension Plans, Supplemental Pension Plan, and Other Postretirement Plan (Detail) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
United States Pension Plans of US Entity, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	$ 10.7	$ 9.5	$ 32.1	$ 28.5
Interest Cost	10.2	9.2	30.6	27.6
Expected Return on Plan Assets	(19.7)	(18.3)	(59.1)	(54.9)
Amortization
Net Loss	6.5	5.0	19.5	15.0
Prior Service Cost	0.4	0.4	1.2	1.2
Net Periodic Benefit Expense (Benefit)	8.1	5.8	24.3	17.4
Foreign Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost		0.2		1.8
Interest Cost	1.6	1.9	4.9	5.2
Expected Return on Plan Assets	(2.1)	(2.3)	(6.2)	(6.2)
Amortization
Net Loss	0.1	0.1	0.1	0.5
Gain on Curtailment of Non-U.S. Plan				(2.2)
Net Periodic Benefit Expense (Benefit)	(0.4)	(0.1)	(1.2)	(0.9)
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.8	0.8	2.4	2.4
Interest Cost	1.1	1.2	3.3	3.6
Amortization
Net Loss	1.4	1.5	4.2	4.5
Prior Service Cost	0.1		0.3
Net Periodic Benefit Expense (Benefit)	3.4	3.5	10.2	10.5
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service Cost	0.1	0.2	0.3	0.6
Interest Cost	0.7	0.7	2.1	2.1
Amortization
Net Loss	0.4	0.5	1.2	1.5
Prior Service Cost	(1.3)	(1.3)	(3.9)	(3.9)
Net Periodic Benefit Expense (Benefit)	$ (0.1)	$ 0.1	$ (0.3)	$ 0.3