Reporting Segments and Related Information - Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 4,361.8	$ 4,432.6	$ 4,361.1
Foreign Exchange Trading Income	203.9	288.6	292.6
Other Noninterest Income	225.8	152.8	428.1
Total Noninterest Income	4,791.5	4,874.0	5,081.8
Net Interest Income	1,982.0	1,887.2	1,382.7
Revenue	6,773.5	6,761.2	6,464.5
Provision for Credit Losses	24.5	12.0	(81.5)
Noninterest Expense	5,284.2	4,982.9	4,535.9
Income (Loss) before Income Taxes	1,464.8	1,766.3	2,010.1
Provision (Benefit) for Income Taxes	357.5	430.3	464.8
NET INCOME	1,107.3	1,336.0	1,545.3
Corporate, Non-Segment
Noninterest Income
Total Noninterest Income	(188.4)	(235.6)	(21.3)
Net Interest Income	0.0	0.0	0.0
Revenue	(188.4)	(235.6)	(21.3)
Noninterest Expense	128.7	74.7	21.8
Income (Loss) before Income Taxes	(317.1)	(310.3)	(43.1)
Provision (Benefit) for Income Taxes	(18.0)	(77.3)	(10.7)
NET INCOME	$ (299.1)	$ (233.0)	$ (32.4)
Percentage of Consolidated Net Income	(27.00%)	(17.00%)	(2.00%)
Average Assets	$ 0.0	$ 0.0	$ 0.0
Segment Reconciling Items
Noninterest Income
Average Assets	142,649.2	152,551.9	156,363.2
Corporate and Institutional Services Results of Operations | Operating Segments
Noninterest Income
Trust, Investment and Other Servicing Fees	2,461.9	2,496.3	2,487.3
Foreign Exchange Trading Income	213.0	281.0	279.0
Other Noninterest Income	263.4	250.7	261.2
Total Noninterest Income	2,938.3	3,028.0	3,027.5
Net Interest Income	1,197.3	1,072.7	637.2
Revenue	4,135.6	4,100.7	3,664.7
Provision for Credit Losses	0.5	2.4	(33.8)
Noninterest Expense	3,273.2	3,092.7	2,863.0
Income (Loss) before Income Taxes	861.9	1,005.6	835.5
Provision (Benefit) for Income Taxes	187.1	243.2	194.1
NET INCOME	$ 674.8	$ 762.4	$ 641.4
Percentage of Consolidated Net Income	61.00%	57.00%	41.00%
Average Assets	$ 101,472.6	$ 115,646.4	$ 120,883.2
Wealth Management | Operating Segments
Noninterest Income
Trust, Investment and Other Servicing Fees	1,899.9	1,936.3	1,873.8
Foreign Exchange Trading Income	(9.1)	7.6	13.6
Other Noninterest Income	150.8	137.7	188.2
Total Noninterest Income	2,041.6	2,081.6	2,075.6
Net Interest Income	842.2	860.1	781.1
Revenue	2,883.8	2,941.7	2,856.7
Provision for Credit Losses	24.0	9.6	(47.7)
Noninterest Expense	1,882.3	1,815.5	1,651.1
Income (Loss) before Income Taxes	977.5	1,116.6	1,253.3
Provision (Benefit) for Income Taxes	245.9	310.0	317.0
NET INCOME	$ 731.6	$ 806.6	$ 936.3
Percentage of Consolidated Net Income	66.00%	60.00%	61.00%
Average Assets	$ 41,176.6	$ 36,905.5	$ 35,480.0