Northern Trust Corporation (Corporation only) - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	$ 1,209.3	$ 1,492.2	$ 1,556.4
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Change in Accrued Income Taxes	25.4	(70.7)	(130.0)
Other Operating Activities, net	(16.4)	(448.2)	729.9
Net Cash Provided by Operating Activities	1,896.8	2,592.0	1,767.5
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from Sale, Maturity and Redemption of Debt Securities – Available for Sale	8,748.2	11,057.2	8,958.7
Acquisition of a Business, Net of Cash Received	0.0	(10.5)	(104.2)
Other Investing Activities, net	(322.7)	225.1	(873.6)
Net Cash (Used in) Provided by Investing Activities	(29,923.0)	(3,405.0)	4,327.1
CASH FLOWS FROM FINANCING ACTIVITIES
Repayments of Senior Notes	(508.6)	0.0	(314.3)
Redemption of Preferred Stock - Series C	(400.0)	0.0	0.0
Proceeds from Issuance of Preferred Stock - Series E	0.0	392.5	0.0
Treasury Stock Purchased	(299.8)	(1,100.2)	(924.3)
Net Proceeds from Stock Options	19.5	44.0	32.6
Cash Dividends Paid on Common Stock	(584.6)	(529.7)	(405.4)
Cash Dividends Paid on Preferred Stock	(45.9)	(46.4)	(46.4)
Other Financing Activities, net	1.2	(1.0)	1.1
Net Cash Provided by (Used In) Financing Activities	27,871.9	615.9	(5,818.2)
Cash and Due from Banks at Beginning of Year	4,459.2
Cash and Due from Banks at End of Year	4,389.5	4,459.2
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	1,209.3	1,492.2	1,556.4
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Equity in Undistributed Net Income of Subsidiaries	(345.9)	541.7	(362.0)
Change in Prepaid Expenses	398.5	(400.4)	(0.6)
Change in Accrued Income Taxes	3.7	114.1	(141.8)
Other Operating Activities, net	300.3	141.9	125.6
Net Cash Provided by Operating Activities	1,565.9	1,889.5	1,177.6
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from Sale, Maturity and Redemption of Debt Securities – Available for Sale	0.0	0.0	1.0
Investments in and Advances to Subsidiaries, net	(800.0)	540.0	(436.5)
Acquisition of a Business, Net of Cash Received	0.0	0.0	(31.2)
Other Investing Activities, net	1.8	3.7	(3.1)
Net Cash (Used in) Provided by Investing Activities	(798.2)	543.7	(469.8)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes	993.2	498.0	497.9
Repayments of Senior Notes	(508.6)	0.0	0.0
Redemption of Preferred Stock - Series C	(400.0)	0.0	0.0
Proceeds from Issuance of Preferred Stock - Series E	0.0	392.5	0.0
Treasury Stock Purchased	(299.8)	(1,100.2)	(924.3)
Net Proceeds from Stock Options	19.5	44.0	32.6
Cash Dividends Paid on Common Stock	(584.6)	(529.7)	(405.4)
Cash Dividends Paid on Preferred Stock	(45.9)	(46.4)	(46.4)
Other Financing Activities, net	15.4	0.9	2.1
Net Cash Provided by (Used In) Financing Activities	(810.8)	(740.9)	(843.5)
Change in Cash and Due from Banks	(43.1)	1,692.3	(135.7)
Cash and Due from Banks at Beginning of Year	2,559.1	866.8	1,002.5
Cash and Due from Banks at End of Year	$ 2,516.0	$ 2,559.1	$ 866.8