Earnings Contribution of Northern Trust's Business Units (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$ 630.7		$ 575.2
Foreign Exchange Trading Income	59.5		61.9
Other Noninterest Income	60.1		71.9
Net Interest Income (FTE)	233.7	[1]	266.3	[1]
Revenue	984.0	[1]	975.3	[1]
Provision for Credit Losses	5.0		5.0
Noninterest Expense	728.9		723.6
Income before Income Taxes	250.1	[1]	246.7	[1]
Provision for Income Taxes	86.1	[1]	85.5	[1]
Net Income	164.0		161.2
Percentage of Consolidated Net Income	100.00%		100.00%
Average Assets	91,569.3		95,128.1
Corporate and Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	348.7		317.0
Foreign Exchange Trading Income	58.2		57.5
Other Noninterest Income	40.2		48.6
Net Interest Income (FTE)	64.1	[1]	77.0	[1]
Revenue	511.2	[1]	500.1	[1]
Provision for Credit Losses	(2.7)		0.5
Noninterest Expense	398.7		398.0
Income before Income Taxes	115.2	[1]	101.6	[1]
Provision for Income Taxes	35.9	[1]	33.6	[1]
Net Income	79.3		68.0
Percentage of Consolidated Net Income	48.00%		42.00%
Average Assets	51,316.8		49,662.2
Personal Financial Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	282.0		258.2
Foreign Exchange Trading Income	1.3		4.4
Other Noninterest Income	17.2		22.7
Net Interest Income (FTE)	147.8	[1]	161.1	[1]
Revenue	448.3	[1]	446.4	[1]
Provision for Credit Losses	7.7		4.5
Noninterest Expense	301.8		303.7
Income before Income Taxes	138.8	[1]	138.2	[1]
Provision for Income Taxes	52.8	[1]	52.3	[1]
Net Income	86.0		85.9
Percentage of Consolidated Net Income	52.00%		53.00%
Average Assets	22,861.4		23,563.9
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	2.7		0.6
Net Interest Income (FTE)	21.8	[1]	28.2	[1]
Revenue	24.5	[1]	28.8	[1]
Noninterest Expense	28.4		21.9
Income before Income Taxes	(3.9)	[1]	6.9	[1]
Provision for Income Taxes	(2.6)	[1]	(0.4)	[1]
Net Income	(1.3)		7.3
Percentage of Consolidated Net Income			5.00%
Average Assets	$ 17,391.1		$ 21,902.0

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $7.6 million for 2013 and $9.9 million for 2012.