Earnings Contribution of Northern Trust's Business Units (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$ 706.9		$ 657.3		$ 1,386.4		$ 1,288.0
Foreign Exchange Trading Income	52.9		71.3		103.0		130.8
Other Noninterest Income	75.3		71.8		140.5		131.9
Net Interest Income (FTE)	253.4	[1]	228.0	[1]	507.8	[2]	461.7	[2]
Revenue	1,088.5	[1]	1,028.4	[1]	2,137.7	[2]	2,012.4	[2]
Provision for Credit Losses			5.0		3.0		10.0
Noninterest Expense	811.0		729.7		1,579.0		1,458.6
Income (loss) before Income Taxes	277.5	[1]	293.7	[1]	555.7	[2]	543.8	[2]
Provision (Benefit) for Income Taxes	95.6	[1]	102.6	[1]	192.4	[2]	188.7	[2]
Net Income	181.9		191.1		363.3		355.1
Percentage of Consolidated Net Income	100.00%		100.00%		100.00%		100.00%
Average Assets	103,324.1		92,849.6		101,792.3		92,213.0
Corporate & Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	395.4		364.2		774.6		712.9
Foreign Exchange Trading Income	50.7		69.8		98.8		128.0
Other Noninterest Income	47.1		45.6		91.5		85.8
Net Interest Income (FTE)	76.7	[1]	66.0	[1]	150.4	[2]	130.1	[2]
Revenue	569.9	[1]	545.6	[1]	1,115.3	[2]	1,056.8	[2]
Provision for Credit Losses	2.4		1.2		3.6		(1.5)
Noninterest Expense	446.4		396.0		869.9		794.7
Income (loss) before Income Taxes	121.1	[1]	148.4	[1]	241.8	[2]	263.6	[2]
Provision (Benefit) for Income Taxes	36.1	[1]	48.3	[1]	70.9	[2]	84.2	[2]
Net Income	85.0		100.1		170.9		179.4
Percentage of Consolidated Net Income	47.00%		52.00%		47.00%		51.00%
Average Assets	59,995.2		51,976.0		58,631.4		51,648.2
Wealth Management
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	311.5		293.1		611.8		575.1
Foreign Exchange Trading Income	2.2		1.5		4.2		2.8
Other Noninterest Income	23.1		24.3		45.8		41.5
Net Interest Income (FTE)	132.6	[1]	141.2	[1]	267.6	[2]	289.0	[2]
Revenue	469.4	[1]	460.1	[1]	929.4	[2]	908.4	[2]
Provision for Credit Losses	(2.4)		3.8		(0.6)		11.5
Noninterest Expense	329.4		301.8		648.2		603.6
Income (loss) before Income Taxes	142.4	[1]	154.5	[1]	281.8	[2]	293.3	[2]
Provision (Benefit) for Income Taxes	53.6	[1]	58.0	[1]	106.2	[2]	110.8	[2]
Net Income	88.8		96.5		175.6		182.5
Percentage of Consolidated Net Income	49.00%		51.00%		48.00%		51.00%
Average Assets	22,210.8		22,803.8		22,368.8		22,832.4
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	5.1		1.9		3.2		4.6
Net Interest Income (FTE)	44.1	[1]	20.8	[1]	89.8	[2]	42.6	[2]
Revenue	49.2	[1]	22.7	[1]	93.0	[2]	47.2	[2]
Noninterest Expense	35.2		31.9		60.9		60.3
Income (loss) before Income Taxes	14.0	[1]	(9.2)	[1]	32.1	[2]	(13.1)	[2]
Provision (Benefit) for Income Taxes	5.9	[1]	(3.7)	[1]	15.3	[2]	(6.3)	[2]
Net Income	8.1		(5.5)		16.8		(6.8)
Percentage of Consolidated Net Income	4.00%		(3.00%)		5.00%		(2.00%)
Average Assets	$ 21,118.1		$ 18,069.8		$ 20,792.1		$ 17,732.4

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $6.8 million for 2014 and $7.9 million for 2013.

[2] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $15.5 million for both 2014 and 2013.