Earnings Contribution of Northern Trust's Reporting Segments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$ 727.5		$ 679.5
Foreign Exchange Trading Income	71.6		50.1
Other Noninterest Income	74.8		65.2
Net Interest Income	266.8	[1]	254.4	[1]
Revenue	1,140.7	[1]	1,049.2	[1]
Provision for Credit Losses	(4.5)		3.0
Noninterest Expense	789.0		768.0
Income before Income Taxes	356.2	[1]	278.2	[1]
Provision for Income Taxes	125.5	[1]	96.8	[1]
Net Income	230.7		181.4
Percentage of Consolidated Net Income	100.00%		100.00%
Average Assets	107,513.2		100,243.5
Corporate & Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	407.3		379.2
Foreign Exchange Trading Income	67.5		48.1
Other Noninterest Income	41.9		44.4
Net Interest Income	96.0	[1]	73.7	[1]
Revenue	612.7	[1]	545.4	[1]
Provision for Credit Losses	(2.2)		1.2
Noninterest Expense	434.9		423.5
Income before Income Taxes	180.0	[1]	120.7	[1]
Provision for Income Taxes	57.3	[1]	34.8	[1]
Net Income	122.7		85.9
Percentage of Consolidated Net Income	53.00%		47.00%
Average Assets	39,762.4		57,252.4
Wealth Management
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	320.2		300.3
Foreign Exchange Trading Income	4.1		2.0
Other Noninterest Income	28.8		22.7
Net Interest Income	138.3	[1]	135.0	[1]
Revenue	491.4	[1]	460.0	[1]
Provision for Credit Losses	(2.3)		1.8
Noninterest Expense	321.9		318.8
Income before Income Taxes	171.8	[1]	139.4	[1]
Provision for Income Taxes	64.6	[1]	52.6	[1]
Net Income	107.2		86.8
Percentage of Consolidated Net Income	47.00%		48.00%
Average Assets	24,308.8		22,528.7
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	4.1		(1.9)
Net Interest Income	32.5	[1]	45.7	[1]
Revenue	36.6	[1]	43.8	[1]
Noninterest Expense	32.2		25.7
Income before Income Taxes	4.4	[1]	18.1	[1]
Provision for Income Taxes	3.6	[1]	9.4	[1]
Net Income	0.8		8.7
Percentage of Consolidated Net Income			5.00%
Average Assets	$ 43,442.0		$ 20,462.4

[1] Stated on an FTE basis. Total consolidated includes FTE adjustments of $6.2 million for 2015 and $8.7 million for 2014.