Reporting Segments and Related Information - Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 4,361.1	$ 3,995.0	$ 3,852.1
Foreign Exchange Trading Income	292.6	290.4	250.9
Other Noninterest Income	428.1	372.2	292.2
Total Noninterest Income	5,081.8	4,657.6	4,395.2
Net Interest Income	1,418.3	1,477.6	1,710.7
Revenue	6,500.1	6,135.2	6,105.9
Provision for Credit Losses	(81.5)	125.0	(14.5)
Noninterest Expense	4,535.9	4,348.2	4,143.5
Income (Loss) before Income Taxes	2,045.7	1,662.0	1,976.9
Provision (Benefit) for Income Taxes	500.4	452.7	484.7
NET INCOME	1,545.3	1,209.3	1,492.2
Average Assets	156,363.2	136,811.1	117,551.4
Segment reporting information, fully taxable-equivalent adjustment	35.6	34.4	32.8
Corporate and Institutional Services Results of Operations
Noninterest Income
Trust, Investment and Other Servicing Fees	2,487.3	2,321.6	2,211.5
Foreign Exchange Trading Income	279.0	276.3	232.2
Other Noninterest Income	261.2	222.5	178.2
Total Noninterest Income	3,027.5	2,820.4	2,621.9
Net Interest Income	637.2	665.5	918.7
Revenue	3,664.7	3,485.9	3,540.6
Provision for Credit Losses	(33.8)	38.1	1.9
Noninterest Expense	2,863.0	2,752.7	2,605.5
Income (Loss) before Income Taxes	835.5	695.1	933.2
Provision (Benefit) for Income Taxes	194.1	174.4	219.4
NET INCOME	$ 641.4	$ 520.7	$ 713.8
Percentage of Consolidated Net Income	41.00%	43.00%	48.00%
Average Assets	$ 120,883.2	$ 104,790.6	$ 87,557.1
Wealth Management
Noninterest Income
Trust, Investment and Other Servicing Fees	1,873.8	1,673.4	1,640.6
Foreign Exchange Trading Income	13.6	14.1	18.7
Other Noninterest Income	188.2	168.0	131.1
Total Noninterest Income	2,075.6	1,855.5	1,790.4
Net Interest Income	781.1	812.1	792.0
Revenue	2,856.7	2,667.6	2,582.4
Provision for Credit Losses	(47.7)	86.9	(16.4)
Noninterest Expense	1,651.1	1,559.7	1,531.6
Income (Loss) before Income Taxes	1,253.3	1,021.0	1,067.2
Provision (Benefit) for Income Taxes	317.0	291.8	271.1
NET INCOME	$ 936.3	$ 729.2	$ 796.1
Percentage of Consolidated Net Income	61.00%	60.00%	53.00%
Average Assets	$ 35,480.0	$ 32,020.5	$ 29,994.3
Treasury and Other Results of Operations
Noninterest Income
Total Noninterest Income	(21.3)	(18.3)	(17.1)
Net Interest Income	0.0	0.0	0.0
Revenue	(21.3)	(18.3)	(17.1)
Noninterest Expense	21.8	35.8	6.4
Income (Loss) before Income Taxes	(43.1)	(54.1)	(23.5)
Provision (Benefit) for Income Taxes	(10.7)	(13.5)	(5.8)
NET INCOME	$ (32.4)	$ (40.6)	$ (17.7)
Percentage of Consolidated Net Income	(2.00%)	(3.00%)	(1.00%)
Average Assets	$ 0.0	$ 0.0	$ 0.0