Reporting Segments and Related Information - Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 3,434.3	$ 3,108.1	$ 2,980.5
Foreign Exchange Trading Income	209.9	236.6	261.8
Other Noninterest Income	301.9	382.2	390.2
Net Interest Income (Note)	1,475.0	1,260.0	1,095.4
Noninterest Income	3,946.1	3,726.9	3,632.5
Revenue (Note)	5,421.1	4,986.9	4,727.9
Provision for Credit Losses	(28.0)	(26.0)	(43.0)
Noninterest Expense	3,769.4	3,470.7	3,280.6
Income before Income Taxes (Note)	1,679.7	1,542.2	1,490.3
Provision for Income Taxes (Note)	480.7	509.7	516.5
Net Income	1,199.0	1,032.5	973.8
Average Assets	119,607.4	115,570.3	110,715.1
Fully taxable equivalent basis adjustments	45.8	25.1	25.3
CORPORATE & INSTITUTIONAL SERVICES
Noninterest Income
Trust, Investment and Other Servicing Fees	1,984.6	1,787.8	1,696.9
Foreign Exchange Trading Income	197.9	224.4	249.4
Other Noninterest Income	176.1	147.0	170.5
Net Interest Income (Note)	733.8	565.0	414.4
Revenue (Note)	3,092.4	2,724.2	2,531.2
Provision for Credit Losses	3.4	1.9	(22.6)
Noninterest Expense	2,194.5	2,012.2	1,856.4
Income before Income Taxes (Note)	894.5	710.1	697.4
Provision for Income Taxes (Note)	279.5	212.9	212.8
Net Income	$ 615.0	$ 497.2	$ 484.6
Percentage of Consolidated Net Income	51.00%	48.00%	50.00%
Average Assets	$ 80,105.6	$ 76,194.7	$ 73,598.4
WEALTH MANAGEMENT
Noninterest Income
Trust, Investment and Other Servicing Fees	1,449.7	1,320.3	1,283.6
Foreign Exchange Trading Income	3.1	8.6	12.4
Other Noninterest Income	103.9	105.7	111.8
Net Interest Income (Note)	736.2	651.4	568.1
Revenue (Note)	2,292.9	2,086.0	1,975.9
Provision for Credit Losses	(31.4)	(27.9)	(20.4)
Noninterest Expense	1,405.3	1,315.3	1,291.9
Income before Income Taxes (Note)	919.0	798.6	704.4
Provision for Income Taxes (Note)	347.2	301.1	264.7
Net Income	$ 571.8	$ 497.5	$ 439.7
Percentage of Consolidated Net Income	48.00%	48.00%	45.00%
Average Assets	$ 26,599.9	$ 26,525.0	$ 25,048.7
Treasury and Other
Noninterest Income
Net Interest Income (Note)	5.0	43.6	112.9
Noninterest Income	30.8	133.1	107.9
Revenue (Note)	35.8	176.7	220.8
Noninterest Expense	169.6	143.2	132.3
Income before Income Taxes (Note)	(133.8)	33.5	88.5
Provision for Income Taxes (Note)	(146.0)	(4.3)	39.0
Net Income	$ 12.2	$ 37.8	$ 49.5
Percentage of Consolidated Net Income	1.00%	4.00%	5.00%
Average Assets	$ 12,901.9	$ 12,850.6	$ 12,068.0