Reporting Segments and Related Information - Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 4,432.6	$ 4,361.1	$ 3,995.0
Foreign Exchange Trading Income	288.6	292.6	290.4
Other Noninterest Income	152.8	428.1	372.2
Total Noninterest Income	4,874.0	5,081.8	4,657.6
Net Interest Income	1,887.2	1,382.7	1,443.2
Revenue	6,761.2	6,464.5	6,100.8
Provision for Credit Losses	12.0	(81.5)	125.0
Noninterest Expense	4,982.9	4,535.9	4,348.2
Income (Loss) before Income Taxes	1,766.3	2,010.1	1,627.6
Provision (Benefit) for Income Taxes	430.3	464.8	418.3
NET INCOME	1,336.0	1,545.3	1,209.3
Personal | Residential Real Estate
Noninterest Income
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Unpaid Principal Balance	152,551.9	5.6
Segment Reconciling Items
Noninterest Income
Average Assets		156,363.2	136,811.1
Corporate and Institutional Services Results of Operations
Noninterest Income
Trust, Investment and Other Servicing Fees	2,496.3	2,487.3	2,321.6
Foreign Exchange Trading Income	281.0	279.0	276.3
Other Noninterest Income	250.7	261.2	222.5
Total Noninterest Income	3,028.0	3,027.5	2,820.4
Net Interest Income	1,072.7	637.2	665.5
Revenue	4,100.7	3,664.7	3,485.9
Provision for Credit Losses	2.4	(33.8)	38.1
Noninterest Expense	3,092.7	2,863.0	2,752.7
Income (Loss) before Income Taxes	1,005.6	835.5	695.1
Provision (Benefit) for Income Taxes	243.2	194.1	174.4
NET INCOME	$ 762.4	$ 641.4	$ 520.7
Percentage of Consolidated Net Income	57.00%	41.00%	43.00%
Average Assets	$ 115,646.4	$ 120,883.2	$ 104,790.6
Wealth Management
Noninterest Income
Trust, Investment and Other Servicing Fees	1,936.3	1,873.8	1,673.4
Foreign Exchange Trading Income	7.6	13.6	14.1
Other Noninterest Income	137.7	188.2	168.0
Total Noninterest Income	2,081.6	2,075.6	1,855.5
Net Interest Income	860.1	781.1	812.1
Revenue	2,941.7	2,856.7	2,667.6
Provision for Credit Losses	9.6	(47.7)	86.9
Noninterest Expense	1,815.5	1,651.1	1,559.7
Income (Loss) before Income Taxes	1,116.6	1,253.3	1,021.0
Provision (Benefit) for Income Taxes	310.0	317.0	291.8
NET INCOME	$ 806.6	$ 936.3	$ 729.2
Percentage of Consolidated Net Income	60.00%	61.00%	60.00%
Average Assets	$ 36,905.5	$ 35,480.0	$ 32,020.5
Treasury and Other Results of Operations
Noninterest Income
Total Noninterest Income	(235.6)	(21.3)	(18.3)
Net Interest Income	0.0	0.0	0.0
Revenue	(235.6)	(21.3)	(18.3)
Noninterest Expense	74.7	21.8	35.8
Income (Loss) before Income Taxes	(310.3)	(43.1)	(54.1)
Provision (Benefit) for Income Taxes	(77.3)	(10.7)	(13.5)
NET INCOME	$ (233.0)	$ (32.4)	$ (40.6)
Percentage of Consolidated Net Income	(17.00%)	(2.00%)	(3.00%)
Average Assets	$ 0.0	$ 0.0	$ 0.0