Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 2,980.5	$ 2,832.8	$ 2,609.8
Foreign Exchange Trading Income	261.8	210.1	244.4
Other Noninterest Income	390.2	282.8	302.0
Net Interest Income (Note)	1,095.4	1,034.9	965.6
Noninterest Income	3,632.5	3,325.7	3,156.2
Revenue (Note)	4,727.9	4,360.6	4,121.8
Provision for Credit Losses	(43.0)	6.0	20.0
Noninterest Expense	3,280.6	3,135.0	2,993.8
Income before Income Taxes (Note)	1,490.3	1,219.6	1,108.0
Provision for Income Taxes (Note)	516.5	407.8	376.7
Net Income	973.8	811.8	731.3
Average Assets	110,715.1	104,083.5	94,857.7
Fully taxable equivalent basis adjustments	25.3	29.4	32.5
Corporate and Institutional Services
Noninterest Income
Trust, Investment and Other Servicing Fees	1,696.9	1,584.0	1,443.8
Foreign Exchange Trading Income	249.4	200.4	238.8
Other Noninterest Income	170.5	177.9	177.3
Net Interest Income (Note)	414.4	310.0	275.9
Revenue (Note)	2,531.2	2,272.3	2,135.8
Provision for Credit Losses	(22.6)	5.8	(3.4)
Noninterest Expense	1,856.4	1,732.8	1,657.9
Income before Income Taxes (Note)	697.4	533.7	481.3
Provision for Income Taxes (Note)	212.8	149.4	145.6
Net Income	$ 484.6	$ 384.3	$ 335.7
Percentage of Consolidated Net Income	50.00%	47.00%	46.00%
Average Assets	$ 42,313.9	$ 59,462.9	$ 53,308.2
Wealth Management
Noninterest Income
Trust, Investment and Other Servicing Fees	1,283.6	1,248.8	1,166.0
Foreign Exchange Trading Income	12.4	9.7	5.6
Other Noninterest Income	111.8	98.3	116.7
Net Interest Income (Note)	568.1	536.1	557.7
Revenue (Note)	1,975.9	1,892.9	1,846.0
Provision for Credit Losses	(20.4)	0.2	23.4
Noninterest Expense	1,291.9	1,268.7	1,215.0
Income before Income Taxes (Note)	704.4	624.0	607.6
Provision for Income Taxes (Note)	264.7	234.8	229.2
Net Income	$ 439.7	$ 389.2	$ 378.4
Percentage of Consolidated Net Income	45.00%	48.00%	52.00%
Average Assets	$ 25,048.7	$ 23,629.3	$ 22,887.6
Treasury and Other
Noninterest Income
Net Interest Income (Note)	112.9	188.8	132.0
Noninterest Income	107.9	6.6	8.0
Revenue (Note)	220.8	195.4	140.0
Noninterest Expense	132.3	133.5	120.9
Income before Income Taxes (Note)	88.5	61.9	19.1
Provision for Income Taxes (Note)	39.0	23.6	1.9
Net Income	$ 49.5	$ 38.3	$ 17.2
Percentage of Consolidated Net Income	5.00%	5.00%	2.00%
Average Assets	$ 43,352.5	$ 20,991.3	$ 18,661.9