Earnings Contribution of Northern Trust's Business Units (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	$ 575.2		$ 514.9
Other	133.8		148.6
Net Interest Income (FTE)	266.3	[1]	244.9	[1]
Revenues	975.3	[1]	908.4	[1]
Provision for Credit Losses	5.0		15.0
Noninterest Expenses	723.6		652.9
Income before Income Taxes	246.7	[1]	240.5	[1]
Provision for Income Taxes	85.5	[1]	89.5	[1]
Net Income	161.2		151.0
Percentage of Consolidated Net Income	100.00%		100.00%
Average Assets	95,128.1		83,265.7
Corporate and Institutional Services
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	317.0		271.3
Other	106.1		124.3
Net Interest Income (FTE)	77.0	[1]	61.7	[1]
Revenues	500.1	[1]	457.3	[1]
Provision for Credit Losses	0.5		(14.6)
Noninterest Expenses	398.0		339.9
Income before Income Taxes	101.6	[1]	132.0	[1]
Provision for Income Taxes	33.6	[1]	49.8	[1]
Net Income	68.0		82.2
Percentage of Consolidated Net Income	42.00%		54.00%
Average Assets	49,662.2		43,710.7
Personal Financial Services
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	258.2		243.6
Other	27.1		31.6
Net Interest Income (FTE)	161.1	[1]	149.2	[1]
Revenues	446.4	[1]	424.4	[1]
Provision for Credit Losses	4.5		29.6
Noninterest Expenses	303.7		290.0
Income before Income Taxes	138.2	[1]	104.8	[1]
Provision for Income Taxes	52.3	[1]	41.7	[1]
Net Income	85.9		63.1
Percentage of Consolidated Net Income	53.00%		42.00%
Average Assets	23,563.9		23,630.3
Treasury and Other
Segment Reporting Information [Line Items]
Other	0.6		(7.3)
Net Interest Income (FTE)	28.2	[1]	34.0	[1]
Revenues	28.8	[1]	26.7	[1]
Noninterest Expenses	21.9		23.0
Income before Income Taxes	6.9	[1]	3.7	[1]
Provision for Income Taxes	(0.4)	[1]	(2.0)	[1]
Net Income	7.3		5.7
Percentage of Consolidated Net Income	5.00%		4.00%
Average Assets	$ 21,902.0		$ 15,924.7

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $9.9 million for 2012 and $10.5 million for 2011.