Northern Trust Corporation (Corporation only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
ASSETS
Cash on Deposit with Subsidiary Bank	$ 4,677.2	$ 4,791.5
Other Assets	13,920.5	10,788.4
Total Assets	155,508.4	150,783.1	$ 155,036.7
LIABILITIES
Senior Notes	2,769.7	2,773.2
Other Liabilities	4,243.8	5,485.1
Total Liabilities	142,720.0	138,885.2
STOCKHOLDERS' EQUITY
Common Stock	408.6	408.6
Additional Paid-In Capital	1,025.3	1,009.6
Retained Earnings	15,614.7	14,233.8
Accumulated Other Comprehensive Loss	(814.0)	(1,137.9)
Treasury Stock	(4,331.1)	(3,501.1)
Total Stockholders’ Equity	12,788.4	11,897.9	11,259.5	$ 12,016.8
Total Liabilities and Stockholders’ Equity	155,508.4	150,783.1
Parent Company
ASSETS
Cash on Deposit with Subsidiary Bank	2,383.1	795.5	$ 601.4	$ 1,731.7
Other Assets	987.3	1,233.1
Total Assets	18,143.5	17,589.7
LIABILITIES
Senior Notes	2,769.7	2,773.2
Long-Term Debt	2,081.3	2,065.0
Other Liabilities	504.1	853.6
Total Liabilities	5,355.1	5,691.8
STOCKHOLDERS' EQUITY
Preferred Stock	884.9	884.9
Common Stock	408.6	408.6
Additional Paid-In Capital	1,025.3	1,009.6
Retained Earnings	15,614.7	14,233.8
Accumulated Other Comprehensive Loss	(814.0)	(1,137.9)
Treasury Stock	(4,331.1)	(3,501.1)
Total Stockholders’ Equity	12,788.4	11,897.9
Total Liabilities and Stockholders’ Equity	18,143.5	17,589.7
Parent Company | Banks Industry
ASSETS
Advances to Wholly-Owned Subsidiaries – Banks	3,760.0	3,760.0
Investments in Wholly-Owned Subsidiaries	10,800.3	11,591.4
Parent Company | Nonbank Subsidiaries
ASSETS
Investments in Wholly-Owned Subsidiaries	$ 212.8	$ 209.7