Earnings Contribution of Northern Trust's Business Units (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$ 657.3		$ 605.8		$ 1,288.0		$ 1,181.0
Foreign Exchange Trading Income	71.3		59.4		130.8		121.3
Other Noninterest Income	71.8		69.2		131.9		141.1
Net Interest Income (FTE)	228.0	[1]	264.3	[1]	461.7	[2]	530.6	[2]
Revenue	1,028.4	[1]	998.7	[1]	2,012.4	[2]	1,974.0	[2]
Provision for Credit Losses	5.0		5.0		10.0		10.0
Noninterest Expense	729.7		717.3		1,458.6		1,440.9
Income (loss) before Income Taxes	293.7	[1]	276.4	[1]	543.8	[2]	523.1	[2]
Provision for Income Taxes	102.6	[1]	96.8	[1]	188.7	[2]	182.3	[2]
Net Income	191.1		179.6		355.1		340.8
Percentage of Consolidated Net Income	100.00%		100.00%		100.00%		100.00%
Average Assets	92,849.6		92,410.6		92,213.0		93,769.3
Corporate and Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	364.2		338.4		712.9		655.4
Foreign Exchange Trading Income	69.8		56.5		128.0		114.0
Other Noninterest Income	45.6		47.1		85.8		95.7
Net Interest Income (FTE)	66.0	[1]	72.0	[1]	130.1	[2]	149.0	[2]
Revenue	545.6	[1]	514.0	[1]	1,056.8	[2]	1,014.1	[2]
Provision for Credit Losses	1.2		(0.5)		(1.5)
Noninterest Expense	396.0		397.5		794.7		795.5
Income (loss) before Income Taxes	148.4	[1]	117.0	[1]	263.6	[2]	218.6	[2]
Provision for Income Taxes	48.3	[1]	35.8	[1]	84.2	[2]	69.4	[2]
Net Income	100.1		81.2		179.4		149.2
Percentage of Consolidated Net Income	52.00%		45.00%		51.00%		44.00%
Average Assets	51,976.0		48,616.8		51,648.2		49,139.5
Personal Financial Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	293.1		267.4		575.1		525.6
Foreign Exchange Trading Income	1.5		2.9		2.8		7.3
Other Noninterest Income	24.3		24.1		41.5		46.8
Net Interest Income (FTE)	141.2	[1]	158.4	[1]	289.0	[2]	319.5	[2]
Revenue	460.1	[1]	452.8	[1]	908.4	[2]	899.2	[2]
Provision for Credit Losses	3.8		5.5		11.5		10.0
Noninterest Expense	301.8		289.5		603.6		593.2
Income (loss) before Income Taxes	154.5	[1]	157.8	[1]	293.3	[2]	296.0	[2]
Provision for Income Taxes	58.0	[1]	59.8	[1]	110.8	[2]	112.1	[2]
Net Income	96.5		98.0		182.5		183.9
Percentage of Consolidated Net Income	51.00%		55.00%		51.00%		54.00%
Average Assets	22,803.8		23,488.3		22,832.4		23,526.1
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	1.9		(2.0)		4.6		(1.4)
Net Interest Income (FTE)	20.8	[1]	33.9	[1]	42.6	[2]	62.1	[2]
Revenue	22.7	[1]	31.9	[1]	47.2	[2]	60.7	[2]
Noninterest Expense	31.9		30.3		60.3		52.2
Income (loss) before Income Taxes	(9.2)	[1]	1.6	[1]	(13.1)	[2]	8.5	[2]
Provision for Income Taxes	(3.7)	[1]	1.2	[1]	(6.3)	[2]	0.8	[2]
Net Income	(5.5)		0.4		(6.8)		7.7
Percentage of Consolidated Net Income	(3.00%)				(2.00%)		2.00%
Average Assets	$ 18,069.8		$ 20,305.5		$ 17,732.4		$ 21,103.7

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $7.9 million for 2013 and $10.2 million for 2012.

[2] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $15.5 million for 2013 and $20.1 million for 2012.