Northern Trust Corporation (Corporation only) - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	$ 1,107.3	$ 1,336.0	$ 1,545.3
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Change in Accrued Income Taxes	(44.4)	61.3	37.2
Other Operating Activities, net	774.5	(333.7)	183.8
Net Cash Provided by Operating Activities	2,625.6	2,392.4	1,356.0
CASH FLOWS FROM INVESTING ACTIVITIES
Other Investing Activities, net	(1,539.0)	(319.3)	(163.7)
Net Cash Provided by (Used in) Investing Activities	4,784.1	25,929.8	(18,602.6)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes	0.0	1,988.8	0.0
Repayments of Senior Notes	0.0	(500.0)	(500.0)
Repayment of Floating Rate Capital Debt	0.0	0.0	(278.8)
Treasury Stock Purchased	(347.5)	(35.4)	(267.6)
Net Proceeds from Stock Options	2.3	3.9	53.8
Cash Dividends Paid on Common Stock	(621.5)	(750.2)	(583.3)
Cash Dividends Paid on Preferred Stock	(41.8)	(46.5)	(41.8)
Other Financing Activities, net	(2.7)	0.0	0.1
Net Cash (Used in) Provided by Financing Activities	(7,182.6)	(26,437.4)	16,073.5
Change in Cash and Due from Banks	137.3	1,597.4	(1,332.7)
Cash on Deposit with Subsidiary Bank at Beginning of Year	4,654.2
Cash on Deposit with Subsidiary Bank at End of Year	4,791.5	4,654.2
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	1,107.3	1,336.0	1,545.3
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Equity in Undistributed Net Income of Subsidiaries	(284.3)	(1,356.2)	(815.8)
Change in Prepaid Expenses	1.7	(3.8)	0.1
Change in Accrued Income Taxes	(10.2)	36.6	2.2
Other Operating Activities, net	138.1	(608.2)	236.6
Net Cash Provided by Operating Activities	952.6	(595.6)	968.4
CASH FLOWS FROM INVESTING ACTIVITIES
Investments in and Advances to Subsidiaries, net	250.0	(1,200.1)	(140.0)
Other Investing Activities, net	0.0	(0.1)	5.1
Net Cash Provided by (Used in) Investing Activities	250.0	(1,200.2)	(134.9)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes	0.0	1,988.8	0.0
Repayments of Senior Notes	0.0	(500.0)	(500.0)
Repayment of Floating Rate Capital Debt	0.0	0.0	(278.8)
Treasury Stock Purchased	(347.5)	(35.4)	(267.6)
Net Proceeds from Stock Options	2.3	3.9	53.8
Cash Dividends Paid on Common Stock	(621.5)	(750.2)	(583.3)
Cash Dividends Paid on Preferred Stock	(41.8)	(46.5)	(41.8)
Other Financing Activities, net	0.0	4.9	(0.1)
Net Cash (Used in) Provided by Financing Activities	(1,008.5)	665.5	(1,617.8)
Change in Cash and Due from Banks	194.1	(1,130.3)	(784.3)
Cash on Deposit with Subsidiary Bank at Beginning of Year	601.4	1,731.7	2,516.0
Cash on Deposit with Subsidiary Bank at End of Year	$ 795.5	$ 601.4	$ 1,731.7