Northern Trust Corporation (Corporation only) - Condensed Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING INCOME
Total Operating Income	$ 5,960.2	$ 5,375.3	$ 4,961.8
OPERATING EXPENSES
Interest Expense	698.7	340.2	182.0
Other Operating Expenses	330.6	331.6	364.4
Total Noninterest Expense	4,016.9	3,769.4	3,470.7
Benefit for Income Taxes	(401.4)	(434.9)	(484.6)
NET INCOME	1,556.4	1,199.0	1,032.5
Preferred Stock Dividends	46.4	49.8	23.4
Net Income Applicable to Common Stock	1,510.0	1,149.2	1,009.1
Parent Company
OPERATING INCOME
Intercompany Interest and Other Charges	91.9	58.2	39.8
Interest and Other Income	(8.7)	18.1	7.5
Total Operating Income	1,284.1	601.3	350.7
OPERATING EXPENSES
Interest Expense	97.3	76.5	63.5
Other Operating Expenses	17.0	25.9	19.9
Total Noninterest Expense	114.3	102.4	83.4
Income before Income Taxes and Equity in Undistributed Net Income of Subsidiaries	1,169.8	498.9	267.3
Benefit for Income Taxes	24.6	43.7	28.3
Income before Equity in Undistributed Net Income of Subsidiaries	1,194.4	542.6	295.6
Equity in Undistributed Net Income of Subsidiaries	362.0	656.4	736.9
NET INCOME	1,556.4	1,199.0	1,032.5
Preferred Stock Dividends	46.4	49.8	23.4
Net Income Applicable to Common Stock	1,510.0	1,149.2	1,009.1
Banks Industry | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	1,200.9	525.0	300.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	336.7	632.6	708.3
Nonbank Subsidiaries | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	0.0	0.0	3.4
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	$ 25.3	$ 23.8	$ 28.6