Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015		Jun. 30, 2014
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$ 756.8		$ 706.9		$ 1,484.3		$ 1,386.4
Foreign Exchange Trading Income	74.8		52.9		146.4		103.0
Other Noninterest Income	173.1		75.3		247.9		140.5
Net Interest Income (FTE)	257.6	[1]	253.4	[1]	524.4	[2]	507.8	[2]
Revenue	1,262.3	[1]	1,088.5	[1]	2,403.0	[2]	2,137.7	[2]
Provision for Credit Losses	(10.0)				(14.5)		3.0
Noninterest Expense	854.5		811.0		1,643.5		1,579.0
Income before Income Taxes	417.8	[1]	277.5	[1]	774.0	[2]	555.7	[2]
Provision for Income Taxes	148.6	[1]	95.6	[1]	274.1	[2]	192.4	[2]
Net Income	$ 269.2		$ 181.9		$ 499.9		$ 363.3
Percentage of Consolidated Net Income	100.00%		100.00%		100.00%		100.00%
Average Assets	$ 111,691.1		$ 103,324.1		$ 109,613.7		$ 101,792.3
Corporate & Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	432.0		395.4		839.3		774.6
Foreign Exchange Trading Income	71.8		50.7		139.3		98.8
Other Noninterest Income	43.2		47.1		85.1		91.5
Net Interest Income (FTE)	92.7	[1]	76.7	[1]	188.7	[2]	150.4	[2]
Revenue	639.7	[1]	569.9	[1]	1,252.4	[2]	1,115.3	[2]
Provision for Credit Losses	2.0		2.4		(0.2)		3.6
Noninterest Expense	488.2		446.4		923.1		869.9
Income before Income Taxes	149.5	[1]	121.1	[1]	329.5	[2]	241.8	[2]
Provision for Income Taxes	45.3	[1]	36.1	[1]	102.6	[2]	70.9	[2]
Net Income	$ 104.2		$ 85.0		$ 226.9		$ 170.9
Percentage of Consolidated Net Income	39.00%		47.00%		45.00%		47.00%
Average Assets	$ 43,299.8		$ 59,995.2		$ 41,540.9		$ 58,631.4
Wealth Management
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	324.8		311.5		645.0		611.8
Foreign Exchange Trading Income	3.0		2.2		7.1		4.2
Other Noninterest Income	26.8		23.1		55.6		45.8
Net Interest Income (FTE)	141.0	[1]	132.6	[1]	279.3	[2]	267.6	[2]
Revenue	495.6	[1]	469.4	[1]	987.0	[2]	929.4	[2]
Provision for Credit Losses	(12.0)		(2.4)		(14.3)		(0.6)
Noninterest Expense	322.7		329.4		644.6		648.2
Income before Income Taxes	184.9	[1]	142.4	[1]	356.7	[2]	281.8	[2]
Provision for Income Taxes	69.6	[1]	53.6	[1]	134.2	[2]	106.2	[2]
Net Income	$ 115.3		$ 88.8		$ 222.5		$ 175.6
Percentage of Consolidated Net Income	43.00%		49.00%		45.00%		48.00%
Average Assets	$ 24,826.5		$ 22,210.8		$ 24,569.1		$ 22,368.8
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	103.1		5.1		107.2		3.2
Net Interest Income (FTE)	23.9	[1]	44.1	[1]	56.4	[2]	89.8	[2]
Revenue	127.0	[1]	49.2	[1]	163.6	[2]	93.0	[2]
Noninterest Expense	43.6		35.2		75.8		60.9
Income before Income Taxes	83.4	[1]	14.0	[1]	87.8	[2]	32.1	[2]
Provision for Income Taxes	33.7	[1]	5.9	[1]	37.3	[2]	15.3	[2]
Net Income	$ 49.7		$ 8.1		$ 50.5		$ 16.8
Percentage of Consolidated Net Income	18.00%		4.00%		10.00%		5.00%
Average Assets	$ 43,564.8		$ 21,118.1		$ 43,503.7		$ 20,792.1

[1] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $6.4 million for 2015 and $6.8 million for 2014.

[2] Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $12.6 million for 2015 and $15.5 million for 2014.