Northern Trust Corporation (Corporation only) - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions		12 Months Ended
	Oct. 15, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income		$ 1,545.3	$ 1,209.3	$ 1,492.2
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Change in Accrued Income Taxes		37.2	25.4	(70.7)
Other Operating Activities, net		183.8	(16.4)	(448.2)
Net Cash Provided by Operating Activities		1,356.0	1,896.8	2,592.0
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from Sale, Maturity and Redemption of Available for Sale Debt Securities		10,079.0	8,748.2	11,057.2
Acquisition of a Business, Net of Cash Received		0.0	0.0	(10.5)
Other Investing Activities, net		(163.7)	(322.7)	225.1
Net Cash Used in Investing Activities		(18,602.6)	(29,923.0)	(3,405.0)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes		0.0	993.2	498.0
Repayments of Senior Notes		(500.0)	(508.6)	0.0
Repayment of Floating Rate Capital Debt	$ (278.8)	(278.8)	0.0	0.0
Redemption of Preferred Stock - Series C		0.0	(400.0)	0.0
Proceeds from Issuance of Preferred Stock - Series E		0.0	0.0	392.5
Treasury Stock Purchased		(267.6)	(299.8)	(1,100.2)
Net Proceeds from Stock Options		53.8	19.5	44.0
Cash Dividends Paid on Common Stock		(583.3)	(584.6)	(529.7)
Cash Dividends Paid on Preferred Stock		(41.8)	(45.9)	(46.4)
Other Financing Activities, net		0.1	1.2	(1.0)
Net Cash Provided by Financing Activities		16,073.5	27,871.9	615.9
Cash on Deposit with Subsidiary Bank at Beginning of Year		4,389.5
Cash on Deposit with Subsidiary Bank at End of Year		3,056.8	4,389.5
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income		1,545.3	1,209.3	1,492.2
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Equity in Undistributed Net Income of Subsidiaries		(815.8)	(345.9)	541.7
Change in Prepaid Expenses		0.1	398.5	(400.4)
Change in Accrued Income Taxes		2.2	3.7	114.1
Other Operating Activities, net		236.6	300.3	141.9
Net Cash Provided by Operating Activities		968.4	1,565.9	1,889.5
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from Sale, Maturity and Redemption of Available for Sale Debt Securities		0.0	0.0	0.0
Investments in and Advances to Subsidiaries, net		(140.0)	(800.0)	540.0
Acquisition of a Business, Net of Cash Received		0.0	0.0	0.0
Other Investing Activities, net		5.1	1.8	3.7
Net Cash Used in Investing Activities		(134.9)	(798.2)	543.7
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes		0.0	993.2	498.0
Repayments of Senior Notes		(500.0)	(508.6)	0.0
Repayment of Floating Rate Capital Debt		(278.8)	0.0	0.0
Redemption of Preferred Stock - Series C		0.0	(400.0)	0.0
Proceeds from Issuance of Preferred Stock - Series E		0.0	0.0	392.5
Treasury Stock Purchased		(267.6)	(299.8)	(1,100.2)
Net Proceeds from Stock Options		53.8	19.5	44.0
Cash Dividends Paid on Common Stock		(583.3)	(584.6)	(529.7)
Cash Dividends Paid on Preferred Stock		(41.8)	(45.9)	(46.4)
Other Financing Activities, net		(0.1)	15.4	0.9
Net Cash Provided by Financing Activities		(1,617.8)	(810.8)	(740.9)
Net Change in Cash on Deposit with Subsidiary Bank		(784.3)	(43.1)	1,692.3
Cash on Deposit with Subsidiary Bank at Beginning of Year		2,516.0	2,559.1	866.8
Cash on Deposit with Subsidiary Bank at End of Year		$ 1,731.7	$ 2,516.0	$ 2,559.1