Earnings Contribution of Northern Trust's Business Units (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$ 679.5		$ 630.7
Foreign Exchange Trading Income	50.1		59.5
Other Noninterest Income	65.2		60.1
Net Interest Income (FTE)	254.4	[1]	233.7	[1]
Revenue	1,049.2	[1]	984.0	[1]
Provision for Credit Losses	3.0		5.0
Noninterest Expense	768.0		728.9
Income (Loss) before Income Taxes	278.2	[1]	250.1	[1]
Provision (Benefit) for Income Taxes	96.8	[1]	86.1	[1]
Net Income	181.4		164.0
Percentage of Consolidated Net Income	100.00%		100.00%
Average Assets	100,243.5		91,569.3
Corporate & Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	379.2		348.7
Foreign Exchange Trading Income	48.1		58.2
Other Noninterest Income	44.4		40.2
Net Interest Income (FTE)	73.7	[1]	64.1	[1]
Revenue	545.4	[1]	511.2	[1]
Provision for Credit Losses	1.2		(2.7)
Noninterest Expense	423.5		398.7
Income (Loss) before Income Taxes	120.7	[1]	115.2	[1]
Provision (Benefit) for Income Taxes	34.8	[1]	35.9	[1]
Net Income	85.9		79.3
Percentage of Consolidated Net Income	47.00%		48.00%
Average Assets	57,252.4		51,316.8
Wealth Management
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	300.3		282.0
Foreign Exchange Trading Income	2.0		1.3
Other Noninterest Income	22.7		17.2
Net Interest Income (FTE)	135.0	[1]	147.8	[1]
Revenue	460.0	[1]	448.3	[1]
Provision for Credit Losses	1.8		7.7
Noninterest Expense	318.8		301.8
Income (Loss) before Income Taxes	139.4	[1]	138.8	[1]
Provision (Benefit) for Income Taxes	52.6	[1]	52.8	[1]
Net Income	86.8		86.0
Percentage of Consolidated Net Income	48.00%		52.00%
Average Assets	22,528.7		22,861.4
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	(1.9)		2.7
Net Interest Income (FTE)	45.7	[1]	21.8	[1]
Revenue	43.8	[1]	24.5	[1]
Noninterest Expense	25.7		28.4
Income (Loss) before Income Taxes	18.1	[1]	(3.9)	[1]
Provision (Benefit) for Income Taxes	9.4	[1]	(2.6)	[1]
Net Income	8.7		(1.3)
Percentage of Consolidated Net Income	5.00%
Average Assets	$ 20,462.4		$ 17,391.1

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $8.7 million for 2014 and $7.6 million for 2013.