Earnings Contribution of Northern Trust's Business Units (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$ 718.2		$ 648.0		$ 2,104.6		$ 1,936.0
Foreign Exchange Trading Income	46.4		62.8		149.4		193.6
Other Noninterest Income	65.0		99.4		205.5		231.3
Net Interest Income	256.2	[1]	244.8	[1]	764.0	[2]	706.5	[2]
Revenue	1,085.8	[1]	1,055.0	[1]	3,223.5	[2]	3,067.4	[2]
Provision for Credit Losses			5.0		3.0		15.0
Noninterest Expense	774.7		740.7		2,353.7		2,199.3
Income (loss) before Income Taxes	311.1	[1]	309.3	[1]	866.8	[2]	853.1	[2]
Provision (Benefit) for Income Taxes	106.6	[1]	102.8	[1]	299.0	[2]	291.5	[2]
Net Income	204.5		206.5		567.8		561.6
Percentage of Consolidated Net Income	100.00%		100.00%		100.00%		100.00%
Average Assets	105,244.7		95,212.5		102,955.8		93,223.5
Corporate & Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	399.9		359.8		1,174.5		1,072.7
Foreign Exchange Trading Income	44.4		61.8		143.2		189.8
Other Noninterest Income	42.8		44.3		134.3		130.1
Net Interest Income	78.4	[1]	70.1	[1]	228.8	[2]	200.2	[2]
Revenue	565.5	[1]	536.0	[1]	1,680.8	[2]	1,592.8	[2]
Provision for Credit Losses	0.9		0.4		4.5		(1.1)
Noninterest Expense	429.6		412.0		1,299.5		1,206.7
Income (loss) before Income Taxes	135.0	[1]	123.6	[1]	376.8	[2]	387.2	[2]
Provision (Benefit) for Income Taxes	42.4	[1]	38.8	[1]	113.3	[2]	123.0	[2]
Net Income	92.6		84.8		263.5		264.2
Percentage of Consolidated Net Income	45.00%		41.00%		46.00%		47.00%
Average Assets	59,907.1		53,653.5		59,061.3		52,323.8
Wealth Management
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	318.3		288.2		930.1		863.3
Foreign Exchange Trading Income	2.0		1.0		6.2		3.8
Other Noninterest Income	20.9		54.3		66.7		95.8
Net Interest Income	131.2	[1]	136.3	[1]	398.8	[2]	425.3	[2]
Revenue	472.4	[1]	479.8	[1]	1,401.8	[2]	1,388.2	[2]
Provision for Credit Losses	(0.9)		4.6		(1.5)		16.1
Noninterest Expense	312.1		297.3		960.3		900.9
Income (loss) before Income Taxes	161.2	[1]	177.9	[1]	443.0	[2]	471.2	[2]
Provision (Benefit) for Income Taxes	60.7	[1]	67.0	[1]	166.9	[2]	177.8	[2]
Net Income	100.5		110.9		276.1		293.4
Percentage of Consolidated Net Income	49.00%		54.00%		49.00%		52.00%
Average Assets	26,061.8		22,923.6		23,613.4		22,863.1
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	1.3		0.8		4.5		5.4
Net Interest Income	46.6	[1]	38.4	[1]	136.4	[2]	81.0	[2]
Revenue	47.9	[1]	39.2	[1]	140.9	[2]	86.4	[2]
Noninterest Expense	33.0		31.4		93.9		91.7
Income (loss) before Income Taxes	14.9	[1]	7.8	[1]	47.0	[2]	(5.3)	[2]
Provision (Benefit) for Income Taxes	3.5	[1]	(3.0)	[1]	18.8	[2]	(9.3)	[2]
Net Income	11.4		10.8		28.2		4.0
Percentage of Consolidated Net Income	6.00%		5.00%		5.00%		1.00%
Average Assets	$ 19,275.8		$ 18,635.4		$ 20,281.1		$ 18,036.6

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $6.9 million for 2014 and $7.8 million for 2013.

[2] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $22.4 million for 2014 and $23.3 million for 2013.