Northern Trust Corporation (Corporation only) - Condensed Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
OPERATING INCOME
Total Operating Income	$ 8,290.4	$ 6,773.5	$ 6,761.2
OPERATING EXPENSES
Interest Expense	7,585.2	5,343.0	990.5
Other Operating Expenses	455.2	472.9	359.3
Total Noninterest Expense	5,633.9	5,284.2	4,982.9
Benefit (Expense) for Income Taxes	(628.4)	(357.5)	(430.3)
NET INCOME	2,031.1	1,107.3	1,336.0
Preferred Stock Dividends	41.8	41.8	41.8
Net Income Applicable to Common Stock	1,989.3	1,065.5	1,294.2
Parent Company
OPERATING INCOME
Intercompany Interest and Other Charges	291.1	260.2	112.8
Interest and Other Income	75.3	12.2	(5.7)
Total Operating Income	3,487.0	1,122.4	107.1
OPERATING EXPENSES
Interest Expense	302.8	279.5	132.3
Other Operating Expenses	47.5	32.3	11.9
Total Noninterest Expense	350.3	311.8	144.2
Income (Loss) before Income Taxes and Equity in Undistributed Net Income of Subsidiaries	3,136.7	810.6	(37.1)
Benefit (Expense) for Income Taxes	(4.1)	12.4	16.9
Income (Loss) before Equity in Undistributed Net Income of Subsidiaries	3,132.6	823.0	(20.2)
Equity in Undistributed Net Income of Subsidiaries	(1,101.5)	284.3	1,356.2
NET INCOME	2,031.1	1,107.3	1,336.0
Preferred Stock Dividends	41.8	41.8	41.8
Net Income Applicable to Common Stock	1,989.3	1,065.5	1,294.2
Banks Industry | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	3,100.6	850.0	0.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	(1,104.6)	269.4	1,338.5
Nonbank Subsidiaries | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	20.0	0.0	0.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	$ 3.1	$ 14.9	$ 17.7