Northern Trust Corporation (Corporation only) - Condensed Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
OPERATING INCOME
Total Operating Income	$ 6,773.5	$ 6,761.2	$ 6,464.5
OPERATING EXPENSES
Interest Expense	5,343.0	990.5	23.8
Other Operating Expenses	472.9	359.3	299.1
Total Noninterest Expense	5,284.2	4,982.9	4,535.9
Benefit for Income Taxes	(357.5)	(430.3)	(464.8)
NET INCOME	1,107.3	1,336.0	1,545.3
Preferred Stock Dividends	41.8	41.8	41.8
Net Income Applicable to Common Stock	1,065.5	1,294.2	1,503.5
Parent Company
OPERATING INCOME
Intercompany Interest and Other Charges	260.2	112.8	20.1
Interest and Other Income	12.2	(5.7)	32.3
Total Operating Income	1,122.4	107.1	811.8
OPERATING EXPENSES
Interest Expense	279.5	132.3	71.3
Other Operating Expenses	32.3	11.9	36.7
Total Noninterest Expense	311.8	144.2	108.0
Income (Loss) before Income Taxes and Equity in Undistributed Net Income of Subsidiaries	810.6	(37.1)	703.8
Benefit for Income Taxes	12.4	16.9	25.7
Income (Loss) before Equity in Undistributed Net Income of Subsidiaries	823.0	(20.2)	729.5
Equity in Undistributed Net Income of Subsidiaries	284.3	1,356.2	815.8
NET INCOME	1,107.3	1,336.0	1,545.3
Preferred Stock Dividends	41.8	41.8	41.8
Net Income Applicable to Common Stock	1,065.5	1,294.2	1,503.5
Banks Industry | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	850.0	0.0	751.1
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	269.4	1,338.5	803.3
Nonbank Subsidiaries | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	0.0	0.0	8.3
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	$ 14.9	$ 17.7	$ 12.5