Northern Trust Corporation (Corporation only) - Condensed Statement of Income (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING INCOME
Total Operating Income		$ 6,100.8	$ 6,073.1	$ 5,960.2
OPERATING EXPENSES
Interest Expense	$ 698.7	200.3	822.0	698.7
Other Operating Expenses		346.7	329.8	330.6
Total Noninterest Expense		4,348.2	4,143.5	4,016.9
Benefit for Income Taxes		(418.3)	(451.9)	(401.4)
NET INCOME		1,209.3	1,492.2	1,556.4
Preferred Stock Dividends		56.2	46.4	46.4
Net Income Applicable to Common Stock		1,153.1	1,445.8	1,510.0
Parent Company
OPERATING INCOME
Intercompany Interest and Other Charges		46.5	115.1	91.9
Interest and Other Income		19.1	20.2	(8.7)
Total Operating Income		965.6	2,159.8	1,284.1
OPERATING EXPENSES
Interest Expense		104.2	121.6	97.3
Other Operating Expenses		26.2	28.6	17.0
Total Noninterest Expense		130.4	150.2	114.3
Income before Income Taxes and Equity in Undistributed Net Income of Subsidiaries		835.2	2,009.6	1,169.8
Benefit for Income Taxes		28.2	24.3	24.6
Income before Equity in Undistributed Net Income of Subsidiaries		863.4	2,033.9	1,194.4
Equity in Undistributed Net Income of Subsidiaries		345.9	(541.7)	362.0
NET INCOME		1,209.3	1,492.2	1,556.4
Preferred Stock Dividends		56.2	46.4	46.4
Net Income Applicable to Common Stock		1,153.1	1,445.8	1,510.0
Banks Industry | Parent Company
OPERATING INCOME
Dividends from Subsidiaries		900.0	2,024.1	1,200.9
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries		326.0	(559.9)	336.7
Nonbank Subsidiaries | Parent Company
OPERATING INCOME
Dividends from Subsidiaries		0.0	0.4	0.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries		$ 19.9	$ 18.2	$ 25.3