Earnings Contribution of Northern Trust's Business Units (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	$ 601.9		$ 555.3		$ 1,782.9		$ 1,628.0
Other	125.0		159.4		387.4		449.0
Net Interest Income (FTE)	256.9	[1]	266.6	[1]	787.5	[2]	768.1	[2]
Revenue	983.8	[1]	981.3	[1]	2,957.8	[2]	2,845.1	[2]
Provision for Credit Losses	10.0		17.5		20.0		42.5
Noninterest Expense	696.4		701.2		2,137.3		2,059.5
Income before Income Taxes	277.4	[1]	262.6	[1]	800.5	[2]	743.1	[2]
Provision for Income Taxes	98.6	[1]	92.2	[1]	280.9	[2]	269.7	[2]
Net Income	178.8		170.4		519.6		473.4
Percentage of Consolidated Net Income	100.00%		100.00%		100.00%		100.00%
Average Assets	92,709.9		94,029.7		93,413.6		89,924.2
Corporate and Institutional Services
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	334.4		310.9		989.8		890.7
Other	91.5		125.0		301.2		373.8
Net Interest Income (FTE)	68.0	[1]	71.2	[1]	217.0	[2]	199.1	[2]
Revenue	493.9	[1]	507.1	[1]	1,508.0	[2]	1,463.6	[2]
Provision for Credit Losses	(1.6)		(1.9)		(1.6)		(18.8)
Noninterest Expense	394.5		375.8		1,190.0		1,096.2
Income before Income Taxes	101.0	[1]	133.2	[1]	319.6	[2]	386.2	[2]
Provision for Income Taxes	32.7	[1]	50.3	[1]	102.1	[2]	147.4	[2]
Net Income	68.3		82.9		217.5		238.8
Percentage of Consolidated Net Income	38.00%		49.00%		42.00%		50.00%
Average Assets	50,638.6		49,755.5		49,642.8		47,079.8
Personal Financial Services
Segment Reporting Information [Line Items]
Noninterest Income Trust, Investment and Other Servicing Fees	267.5		244.4		793.1		737.3
Other	26.0		30.7		80.1		94.9
Net Interest Income (FTE)	157.4	[1]	157.1	[1]	476.9	[2]	456.6	[2]
Revenue	450.9	[1]	432.2	[1]	1,350.1	[2]	1,288.8	[2]
Provision for Credit Losses	11.6		19.4		21.6		61.3
Noninterest Expense	285.2		300.6		878.4		893.3
Income before Income Taxes	154.1	[1]	112.2	[1]	450.1	[2]	334.2	[2]
Provision for Income Taxes	58.1	[1]	44.5	[1]	170.2	[2]	132.7	[2]
Net Income	96.0		67.7		279.9		201.5
Percentage of Consolidated Net Income	54.00%		40.00%		54.00%		43.00%
Average Assets	23,530.7		23,809.5		23,527.6		23,696.1
Treasury and Other
Segment Reporting Information [Line Items]
Other	7.5		3.7		6.1		(19.7)
Net Interest Income (FTE)	31.5	[1]	38.3	[1]	93.6	[2]	112.4	[2]
Revenue	39.0	[1]	42.0	[1]	99.7	[2]	92.7	[2]
Noninterest Expense	16.7		24.8		68.9		70.0
Income before Income Taxes	22.3	[1]	17.2	[1]	30.8	[2]	22.7	[2]
Provision for Income Taxes	7.8	[1]	(2.6)	[1]	8.6	[2]	(10.4)	[2]
Net Income	14.5		19.8		22.2		33.1
Percentage of Consolidated Net Income	8.00%		11.00%		4.00%		7.00%
Average Assets	$ 18,540.6		$ 20,464.7		$ 20,243.2		$ 19,148.3

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $11.3 million for 2012 and $9.8 million for 2011.

[2] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $31.4 million for 2012 and $30.8 million for 2011.