Northern Trust Corporation (Corporation only) - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
OPERATING ACTIVITIES:
Net Income	$ 1,492.2	$ 1,556.4	$ 1,199.0
Adjustments to Reconcile Net Income to Net Cash Provided by (Used in) Operating Activities:
Change in Accrued Income Taxes	(70.7)	(130.0)	36.2
Other Operating Activities, net	(448.2)	729.9	(302.1)
Net Cash Provided by Operating Activities	2,592.0	1,767.5	1,720.4
INVESTING ACTIVITIES:
Proceeds from Sale, Maturity and Redemption of Debt Securities – Available for Sale	11,057.2	8,958.7	10,103.4
Acquisition of a Business, Net of Cash Received	(10.5)	(104.2)	(188.5)
Other Investing Activities, net	225.1	(873.6)	25.8
Net Cash (Used in) Provided by Investing Activities	(3,405.0)	4,327.1	(14,021.6)
FINANCING ACTIVITIES:
Proceeds from Issuance of Preferred Stock - Series E	392.5	0.0	0.0
Treasury Stock Purchased	(1,100.2)	(924.3)	(523.1)
Net Proceeds from Stock Options	44.0	32.6	108.0
Cash Dividends Paid on Common Stock	(529.7)	(405.4)	(356.8)
Cash Dividends Paid on Preferred Stock	(46.4)	(46.4)	(49.8)
Other Financing Activities, net	(1.0)	1.1	0.1
Net Cash Provided by (Used In) Financing Activities	615.9	(5,818.2)	11,252.7
Cash and Due from Banks at Beginning of Year	4,581.6
Cash and Due from Banks at End of Year	4,459.2	4,581.6
Parent Company
OPERATING ACTIVITIES:
Net Income	1,492.2	1,556.4	1,199.0
Adjustments to Reconcile Net Income to Net Cash Provided by (Used in) Operating Activities:
Equity in Undistributed Net Income of Subsidiaries	541.7	(362.0)	(656.4)
Change in Prepaid Expenses	(400.4)	(0.6)	(0.3)
Change in Accrued Income Taxes	114.1	(141.8)	17.2
Other Operating Activities, net	141.9	125.6	55.7
Net Cash Provided by Operating Activities	1,889.5	1,177.6	615.2
INVESTING ACTIVITIES:
Proceeds from Sale, Maturity and Redemption of Debt Securities – Available for Sale	0.0	1.0	0.0
Advances to Wholly-Owned Subsidiaries	540.0	(436.5)	100.0
Acquisition of a Business, Net of Cash Received	0.0	(31.2)	0.0
Other Investing Activities, net	3.7	(3.1)	1.9
Net Cash (Used in) Provided by Investing Activities	543.7	(469.8)	101.9
FINANCING ACTIVITIES:
Proceeds from Senior Notes	498.0	497.9	350.0
Proceeds from Issuance of Preferred Stock - Series E	391.4	0.0	0.0
Treasury Stock Purchased	(1,100.2)	(924.3)	(523.1)
Net Proceeds from Stock Options	44.0	32.6	108.0
Cash Dividends Paid on Common Stock	(529.7)	(405.4)	(356.8)
Cash Dividends Paid on Preferred Stock	(46.4)	(46.4)	(49.8)
Other Financing Activities, net	2.0	2.1	0.1
Net Cash Provided by (Used In) Financing Activities	(740.9)	(843.5)	(471.6)
Change in Cash and Due from Banks	1,692.3	(135.7)	245.5
Cash and Due from Banks at Beginning of Year	866.8	1,002.5	757.0
Cash and Due from Banks at End of Year	$ 2,559.1	$ 866.8	$ 1,002.5