Northern Trust Corporation (Corporation only) - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES:
Net Income	$ 1,556.4	$ 1,199.0	$ 1,032.5
Adjustments to Reconcile Net Income to Net Cash Provided by (Used in) Operating Activities:
Change in Accrued Income Taxes	(130.0)	36.2	(129.0)
Other, net	729.9	(302.1)	653.4
Net Cash Provided by Operating Activities	1,767.5	1,720.4	1,510.0
INVESTING ACTIVITIES:
Proceeds from Sale, Maturity and Redemption of Debt Securities – Available for Sale	8,958.7	10,103.4	11,317.3
Acquisition of a Subsidiary, net of cash acquired	(104.2)	(188.5)	(16.9)
Other, net	(873.6)	25.8	226.5
Net Cash Provided by (Used in) Investing Activities	4,327.1	(14,021.6)	(10,192.1)
FINANCING ACTIVITIES:
Proceeds from Issuance of Preferred Stock - Series D	0.0	0.0	493.5
Treasury Stock Purchased	(924.3)	(523.1)	(411.1)
Net Proceeds from Stock Options	32.6	108.0	233.8
Cash Dividends Paid on Common Stock	(405.4)	(356.8)	(333.0)
Cash Dividends Paid on Preferred Stock	(46.4)	(49.8)	(23.4)
Other, net	1.1	0.1	(7.5)
Net Cash (Used In) Provided by Financing Activities	(5,818.2)	11,252.7	7,536.9
Change in Cash and Due from Banks	63.5	(813.9)	(1,086.5)
Cash and Due from Banks at Beginning of Year	4,518.1	5,332.0	6,418.5
Cash and Due from Banks at End of Year	4,581.6	4,518.1	5,332.0
Parent Company
OPERATING ACTIVITIES:
Net Income	1,556.4	1,199.0	1,032.5
Adjustments to Reconcile Net Income to Net Cash Provided by (Used in) Operating Activities:
Equity in Undistributed Net Income of Subsidiaries	(362.0)	(656.4)	(736.9)
Change in Prepaid Expenses	(0.6)	(0.3)	3.0
Change in Accrued Income Taxes	(141.8)	17.2	(17.9)
Other, net	125.6	55.7	55.7
Net Cash Provided by Operating Activities	1,177.6	615.2	336.4
INVESTING ACTIVITIES:
Proceeds from Sale, Maturity and Redemption of Debt Securities – Available for Sale	1.0	0.0	0.2
Change in Capital Investments in Subsidiaries	0.0	0.0	(3.0)
Advances to Wholly-Owned Subsidiaries	(436.5)	100.0	(295.0)
Acquisition of a Subsidiary, net of cash acquired	(31.2)	0.0	0.0
Other, net	(3.1)	1.9	1.2
Net Cash Provided by (Used in) Investing Activities	(469.8)	101.9	(296.6)
FINANCING ACTIVITIES:
Proceeds from Senior Notes and Long-Term Debt	497.9	350.0	0.0
Proceeds from Issuance of Preferred Stock - Series D	0.0	0.0	493.5
Treasury Stock Purchased	(924.3)	(523.1)	(411.1)
Net Proceeds from Stock Options	32.6	108.0	233.8
Cash Dividends Paid on Common Stock	(405.4)	(356.8)	(333.0)
Cash Dividends Paid on Preferred Stock	(46.4)	(49.8)	(23.4)
Other, net	2.1	0.1	(0.1)
Net Cash (Used In) Provided by Financing Activities	(843.5)	(471.6)	(40.3)
Change in Cash and Due from Banks	(135.7)	245.5	(0.5)
Cash and Due from Banks at Beginning of Year	1,002.5	757.0	757.5
Cash and Due from Banks at End of Year	$ 866.8	$ 1,002.5	$ 757.0