Northern Trust Corporation (Corporation only) - Condensed Statement of Income (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
OPERATING INCOME
Total Operating Income		$ 6,464.5	$ 6,100.8	$ 6,073.1
OPERATING EXPENSES
Interest Expense	$ 822.0	23.8	200.3	822.0
Other Operating Expenses		299.1	346.7	329.8
Total Noninterest Expense		4,535.9	4,348.2	4,143.5
Benefit for Income Taxes		(464.8)	(418.3)	(451.9)
NET INCOME		1,545.3	1,209.3	1,492.2
Preferred Stock Dividends		41.8	56.2	46.4
Net Income Applicable to Common Stock		1,503.5	1,153.1	1,445.8
Parent Company
OPERATING INCOME
Intercompany Interest and Other Charges		20.1	46.5	115.1
Interest and Other Income		32.3	19.1	20.2
Total Operating Income		811.8	965.6	2,159.8
OPERATING EXPENSES
Interest Expense		71.3	104.2	121.6
Other Operating Expenses		36.7	26.2	28.6
Total Noninterest Expense		108.0	130.4	150.2
Income before Income Taxes and Equity in Undistributed Net Income of Subsidiaries		703.8	835.2	2,009.6
Benefit for Income Taxes		25.7	28.2	24.3
Income before Equity in Undistributed Net Income of Subsidiaries		729.5	863.4	2,033.9
Equity in Undistributed Net Income of Subsidiaries		815.8	345.9	(541.7)
NET INCOME		1,545.3	1,209.3	1,492.2
Preferred Stock Dividends		41.8	56.2	46.4
Net Income Applicable to Common Stock		1,503.5	1,153.1	1,445.8
Banks Industry | Parent Company
OPERATING INCOME
Dividends from Subsidiaries		751.1	900.0	2,024.1
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries		803.3	326.0	(559.9)
Nonbank Subsidiaries | Parent Company
OPERATING INCOME
Dividends from Subsidiaries		8.3	0.0	0.4
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries		$ 12.5	$ 19.9	$ 18.2