Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 3,108.1	$ 2,980.5	$ 2,832.8
Foreign Exchange Trading Income	236.6	261.8	210.1
Other Noninterest Income	382.2	390.2	282.8
Net Interest Income (Note)	1,260.0	1,095.4	1,034.9
Noninterest Income	3,726.9	3,632.5	3,325.7
Revenue (Note)	4,986.9	4,727.9	4,360.6
Provision for Credit Losses	(26.0)	(43.0)	6.0
Noninterest Expense	3,470.7	3,280.6	3,135.0
Income before Income Taxes (Note)	1,542.2	1,490.3	1,219.6
Provision for Income Taxes (Note)	509.7	516.5	407.8
Net Income	1,032.5	973.8	811.8
Average Assets	115,570.3	110,715.1	104,083.5
Fully taxable equivalent basis adjustments	25.1	25.3	29.4
Corporate and Institutional Services
Noninterest Income
Trust, Investment and Other Servicing Fees	1,787.8	1,696.9	1,584.0
Foreign Exchange Trading Income	224.4	249.4	200.4
Other Noninterest Income	147.0	170.5	177.9
Net Interest Income (Note)	565.0	414.4	310.0
Revenue (Note)	2,724.2	2,531.2	2,272.3
Provision for Credit Losses	1.9	(22.6)	5.8
Noninterest Expense	2,012.2	1,856.4	1,732.8
Income before Income Taxes (Note)	710.1	697.4	533.7
Provision for Income Taxes (Note)	212.9	212.8	149.4
Net Income	$ 497.2	$ 484.6	$ 384.3
Percentage of Consolidated Net Income	48.00%	50.00%	47.00%
Average Assets	$ 76,194.7	$ 73,598.4	$ 59,462.9
Wealth Management
Noninterest Income
Trust, Investment and Other Servicing Fees	1,320.3	1,283.6	1,248.8
Foreign Exchange Trading Income	8.6	12.4	9.7
Other Noninterest Income	105.7	111.8	98.3
Net Interest Income (Note)	651.4	568.1	536.1
Revenue (Note)	2,086.0	1,975.9	1,892.9
Provision for Credit Losses	(27.9)	(20.4)	0.2
Noninterest Expense	1,315.3	1,291.9	1,268.7
Income before Income Taxes (Note)	798.6	704.4	624.0
Provision for Income Taxes (Note)	301.1	264.7	234.8
Net Income	$ 497.5	$ 439.7	$ 389.2
Percentage of Consolidated Net Income	48.00%	45.00%	48.00%
Average Assets	$ 26,525.0	$ 25,048.7	$ 23,629.3
Treasury and Other
Noninterest Income
Net Interest Income (Note)	43.6	112.9	188.8
Noninterest Income	133.1	107.9	6.6
Revenue (Note)	176.7	220.8	195.4
Noninterest Expense	143.2	132.3	133.5
Income before Income Taxes (Note)	33.5	88.5	61.9
Provision for Income Taxes (Note)	(4.3)	39.0	23.6
Net Income	$ 37.8	$ 49.5	$ 38.3
Percentage of Consolidated Net Income	4.00%	5.00%	5.00%
Average Assets	$ 12,850.6	$ 12,068.0	$ 20,991.3