Earnings Contribution of Northern Trust's Business Units (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	$ 605.8		$ 557.8		$ 1,181.0		$ 1,072.7
Other	128.6		140.9		262.4		289.5
Net Interest Income (FTE)	264.3	[1]	256.6	[1]	530.6	[2]	501.5	[2]
Revenues	998.7	[1]	955.3	[1]	1,974.0	[2]	1,863.7	[2]
Provision for Credit Losses	5.0		10.0		10.0		25.0
Noninterest Expense	717.3		705.3		1,440.9		1,358.2
Income before Income Taxes	276.4	[1]	240.0	[1]	523.1	[2]	480.5	[2]
Provision for Income Taxes	96.8	[1]	88.0	[1]	182.3	[2]	177.5	[2]
Net Income	179.6		152.0		340.8		303.0
Percentage of Consolidated Net Income	100.00%		100.00%		100.00%		100.00%
Average Assets	92,410.6		92,359.1		93,769.3		87,837.4
Corporate and Institutional Services
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	338.4		308.5		655.4		579.8
Other	103.6		124.5		209.7		248.8
Net Interest Income (FTE)	72.0	[1]	66.2	[1]	149.0	[2]	127.9	[2]
Revenues	514.0	[1]	499.2	[1]	1,014.1	[2]	956.5	[2]
Provision for Credit Losses	(0.5)		(2.3)				(16.9)
Noninterest Expense	397.5		380.5		795.5		720.4
Income before Income Taxes	117.0	[1]	121.0	[1]	218.6	[2]	253.0	[2]
Provision for Income Taxes	35.8	[1]	47.3	[1]	69.4	[2]	97.1	[2]
Net Income	81.2		73.7		149.2		155.9
Percentage of Consolidated Net Income	45.00%		48.00%		44.00%		52.00%
Average Assets	48,616.8		47,706.7		49,139.5		45,719.7
Personal Financial Services
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	267.4		249.3		525.6		492.9
Other	27.0		32.6		54.1		64.2
Net Interest Income (FTE)	158.4	[1]	150.3	[1]	319.5	[2]	299.5	[2]
Revenues	452.8	[1]	432.2	[1]	899.2	[2]	856.6	[2]
Provision for Credit Losses	5.5		12.3		10.0		41.9
Noninterest Expense	289.5		302.7		593.2		592.7
Income before Income Taxes	157.8	[1]	117.2	[1]	296.0	[2]	222.0	[2]
Provision for Income Taxes	59.8	[1]	46.5	[1]	112.1	[2]	88.2	[2]
Net Income	98.0		70.7		183.9		133.8
Percentage of Consolidated Net Income	55.00%		47.00%		54.00%		44.00%
Average Assets	23,488.3		23,646.5		23,526.1		23,638.4
Treasury and Other
Segment Reporting Information [Line Items]
Other	(2.0)		(16.2)		(1.4)		(23.5)
Net Interest Income (FTE)	33.9	[1]	40.1	[1]	62.1	[2]	74.1	[2]
Revenues	31.9	[1]	23.9	[1]	60.7	[2]	50.6	[2]
Noninterest Expense	30.3		22.1		52.2		45.1
Income before Income Taxes	1.6	[1]	1.8	[1]	8.5	[2]	5.5	[2]
Provision for Income Taxes	1.2	[1]	(5.8)	[1]	0.8	[2]	(7.8)	[2]
Net Income	0.4		7.6		7.7		13.3
Percentage of Consolidated Net Income			5.00%		2.00%		4.00%
Average Assets	$ 20,305.5		$ 21,005.9		$ 21,103.7		$ 18,479.3

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $10.2 million for 2012 and $10.5 million for 2011.

[2] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $20.1 million for 2012 and $21.0 million for 2011.