Northern Trust Corporation (Corporation only) - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	$ 1,336.0	$ 1,545.3	$ 1,209.3
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Change in Accrued Income Taxes	61.3	37.2	25.4
Other Operating Activities, net	(333.7)	183.8	(16.4)
Net Cash (Used in) Provided by Operating Activities	2,392.4	1,356.0	1,896.8
CASH FLOWS FROM INVESTING ACTIVITIES
Other Investing Activities, net	(319.3)	(163.7)	(322.7)
Net Cash Used in Investing Activities	25,929.8	(18,602.6)	(29,923.0)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes and Long-Term Debt	1,988.8	0.0	993.2
Repayments of Senior Notes	(500.0)	(500.0)	(508.6)
Repayment of Floating Rate Capital Debt	0.0	(278.8)	0.0
Redemption of Preferred Stock - Series C	0.0	0.0	(400.0)
Treasury Stock Purchased	(35.4)	(267.6)	(299.8)
Net Proceeds from Stock Options	3.9	53.8	19.5
Cash Dividends Paid on Common Stock	(750.2)	(583.3)	(584.6)
Cash Dividends Paid on Preferred Stock	(46.5)	(41.8)	(45.9)
Other Financing Activities, net	0.0	0.1	1.2
Net Cash Provided by (Used in) Financing Activities	(26,437.4)	16,073.5	27,871.9
Change in Cash and Due from Banks	1,597.4	(1,332.7)	(69.7)
Cash on Deposit with Subsidiary Bank at Beginning of Year	3,056.8
Cash on Deposit with Subsidiary Bank at End of Year	4,654.2	3,056.8
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
Net Income	1,336.0	1,545.3	1,209.3
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Equity in Undistributed Net Income of Subsidiaries	(1,356.2)	(815.8)	(345.9)
Change in Prepaid Expenses	(3.8)	0.1	398.5
Change in Accrued Income Taxes	36.6	2.2	3.7
Other Operating Activities, net	(608.2)	236.6	300.3
Net Cash (Used in) Provided by Operating Activities	(595.6)	968.4	1,565.9
CASH FLOWS FROM INVESTING ACTIVITIES
Investments in and Advances to Subsidiaries, net	(1,200.1)	(140.0)	(800.0)
Other Investing Activities, net	(0.1)	5.1	1.8
Net Cash Used in Investing Activities	(1,200.2)	(134.9)	(798.2)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from Senior Notes and Long-Term Debt	1,988.8	0.0	993.2
Repayments of Senior Notes	(500.0)	(500.0)	(508.6)
Repayment of Floating Rate Capital Debt	0.0	(278.8)	0.0
Redemption of Preferred Stock - Series C	0.0	0.0	(400.0)
Treasury Stock Purchased	(35.4)	(267.6)	(299.8)
Net Proceeds from Stock Options	3.9	53.8	19.5
Cash Dividends Paid on Common Stock	(750.2)	(583.3)	(584.6)
Cash Dividends Paid on Preferred Stock	(46.5)	(41.8)	(45.9)
Other Financing Activities, net	4.9	(0.1)	15.4
Net Cash Provided by (Used in) Financing Activities	665.5	(1,617.8)	(810.8)
Change in Cash and Due from Banks	(1,130.3)	(784.3)	(43.1)
Cash on Deposit with Subsidiary Bank at Beginning of Year	1,731.7	2,516.0	2,559.1
Cash on Deposit with Subsidiary Bank at End of Year	$ 601.4	$ 1,731.7	$ 2,516.0