Northern Trust Corporation (Corporation only) - Condensed Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
OPERATING INCOME
Total Operating Income	$ 6,073.1	$ 5,960.2	$ 5,375.3
OPERATING EXPENSES
Interest Expense	822.0	698.7	340.2
Other Operating Expenses	329.8	330.6	331.6
Total Noninterest Expense	4,143.5	4,016.9	3,769.4
Benefit for Income Taxes	(451.9)	(401.4)	(434.9)
NET INCOME	1,492.2	1,556.4	1,199.0
Preferred Stock Dividends	46.4	46.4	49.8
Net Income Applicable to Common Stock	1,445.8	1,510.0	1,149.2
Parent Company
OPERATING INCOME
Intercompany Interest and Other Charges	115.1	91.9	58.2
Interest and Other Income	20.2	(8.7)	18.1
Total Operating Income	2,159.8	1,284.1	601.3
OPERATING EXPENSES
Interest Expense	121.6	97.3	76.5
Other Operating Expenses	28.6	17.0	25.9
Total Noninterest Expense	150.2	114.3	102.4
Income before Income Taxes and Equity in Undistributed Net Income of Subsidiaries	2,009.6	1,169.8	498.9
Benefit for Income Taxes	24.3	24.6	43.7
Income before Equity in Undistributed Net Income of Subsidiaries	2,033.9	1,194.4	542.6
Equity in Undistributed Net Income of Subsidiaries	(541.7)	362.0	656.4
NET INCOME	1,492.2	1,556.4	1,199.0
Preferred Stock Dividends	46.4	46.4	49.8
Net Income Applicable to Common Stock	1,445.8	1,510.0	1,149.2
Banks Industry | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	2,024.1	1,200.9	525.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	(559.9)	336.7	632.6
Nonbank Subsidiaries | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	0.4	0.0	0.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	$ 18.2	$ 25.3	$ 23.8