Northern Trust Corporation (Corporation only) - Condensed Statement of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING INCOME
Total Operating Income	$ 5,375.3	$ 4,961.8	$ 4,702.6
OPERATING EXPENSES
Interest Expense	340.2	182.0	153.9
Other Operating Expenses	331.6	364.4	328.0
Total Noninterest Expense	3,769.4	3,470.7	3,280.6
Benefit for Income Taxes	(434.9)	(484.6)	(491.2)
NET INCOME	1,199.0	1,032.5	973.8
Preferred Stock Dividends	49.8	23.4	23.4
Net Income Applicable to Common Stock	1,149.2	1,009.1	950.4
Parent Company
OPERATING INCOME
Intercompany Interest and Other Charges	58.2	39.8	38.5
Interest and Other Income	18.1	7.5	5.4
Total Operating Income	601.3	350.7	652.6
OPERATING EXPENSES
Interest Expense	76.5	63.5	59.3
Other Operating Expenses	25.9	19.9	56.8
Total Noninterest Expense	102.4	83.4	116.1
Income before Income Taxes and Equity in Undistributed Net Income of Subsidiaries	498.9	267.3	536.5
Benefit for Income Taxes	43.7	28.3	29.0
Income before Equity in Undistributed Net Income of Subsidiaries	542.6	295.6	565.5
Equity in Undistributed Net Income of Subsidiaries	656.4	736.9	408.3
NET INCOME	1,199.0	1,032.5	973.8
Preferred Stock Dividends	49.8	23.4	23.4
Net Income Applicable to Common Stock	1,149.2	1,009.1	950.4
Banks Industry | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	525.0	300.0	600.0
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	632.6	708.3	392.8
Nonbank Subsidiaries | Parent Company
OPERATING INCOME
Dividends from Subsidiaries	0.0	3.4	8.7
OPERATING EXPENSES
Equity in Undistributed Net Income of Subsidiaries	$ 23.8	$ 28.6	$ 15.5