Northern Trust Corporation (Corporation only) - Condensed Statement of Income (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
OPERATING INCOME
Total Operating Income		$ 6,761.2	$ 6,464.5	$ 6,100.8
OPERATING EXPENSES
Interest Expense	$ 200.3	990.5	23.8	200.3
Other Operating Expenses		359.3	299.1	346.7
Total Noninterest Expense		4,982.9	4,535.9	4,348.2
Benefit for Income Taxes		(430.3)	(464.8)	(418.3)
NET INCOME		1,336.0	1,545.3	1,209.3
Preferred Stock Dividends		41.8	41.8	56.2
Net Income Applicable to Common Stock		1,294.2	1,503.5	1,153.1
Parent Company
OPERATING INCOME
Intercompany Interest and Other Charges		112.8	20.1	46.5
Interest and Other Income		(5.7)	32.3	19.1
Total Operating Income		107.1	811.8	965.6
OPERATING EXPENSES
Interest Expense		132.3	71.3	104.2
Other Operating Expenses		11.9	36.7	26.2
Total Noninterest Expense		144.2	108.0	130.4
Income (Loss) before Income Taxes and Equity in Undistributed Net Income of Subsidiaries		(37.1)	703.8	835.2
Benefit for Income Taxes		16.9	25.7	28.2
Income (Loss) before Equity in Undistributed Net Income of Subsidiaries		(20.2)	729.5	863.4
Equity in Undistributed Net Income of Subsidiaries		1,356.2	815.8	345.9
NET INCOME		1,336.0	1,545.3	1,209.3
Preferred Stock Dividends		41.8	41.8	56.2
Net Income Applicable to Common Stock		1,294.2	1,503.5	1,153.1
Banks Industry | Parent Company
OPERATING INCOME
Dividends from Subsidiaries		0.0	751.1	900.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries		1,338.5	803.3	326.0
Nonbank Subsidiaries | Parent Company
OPERATING INCOME
Dividends from Subsidiaries		0.0	8.3	0.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries		$ 17.7	$ 12.5	$ 19.9