Northern Trust Corporation (Corporation only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
ASSETS
Cash on Deposit with Subsidiary Bank	$ 4,791.5	$ 4,654.2
Other Assets	10,788.4	9,855.2
Total Assets	150,783.1	155,036.7	$ 183,889.8
LIABILITIES
Senior Notes	2,773.2	2,724.2
Other Liabilities	5,485.1	4,998.3
Total Liabilities	138,885.2	143,777.2
STOCKHOLDERS' EQUITY
Common Stock	408.6	408.6
Additional Paid-In Capital	1,009.6	983.5
Retained Earnings	14,233.8	13,798.5
Accumulated Other Comprehensive Loss	(1,137.9)	(1,569.2)
Treasury Stock	(3,501.1)	(3,246.8)
Total Stockholders’ Equity	11,897.9	11,259.5	12,016.8	$ 11,688.3
Total Liabilities and Stockholders’ Equity	150,783.1	155,036.7
Parent Company
ASSETS
Cash on Deposit with Subsidiary Bank	795.5	601.4	$ 1,731.7	$ 2,516.0
Other Assets	1,233.1	1,226.3
Total Assets	17,589.7	16,929.5
LIABILITIES
Senior Notes	2,773.2	2,724.2
Long-Term Debt	2,065.0	2,066.2
Other Liabilities	853.6	879.6
Total Liabilities	5,691.8	5,670.0
STOCKHOLDERS' EQUITY
Preferred Stock	884.9	884.9
Common Stock	408.6	408.6
Additional Paid-In Capital	1,009.6	983.5
Retained Earnings	14,233.8	13,798.5
Accumulated Other Comprehensive Loss	(1,137.9)	(1,569.2)
Treasury Stock	(3,501.1)	(3,246.8)
Total Stockholders’ Equity	11,897.9	11,259.5
Total Liabilities and Stockholders’ Equity	17,589.7	16,929.5
Parent Company | Banks Industry
ASSETS
Advances to Wholly-Owned Subsidiaries – Banks	3,760.0	4,010.0
Investments in Wholly-Owned Subsidiaries	11,591.4	10,897.1
Parent Company | Nonbank Subsidiaries
ASSETS
Investments in Wholly-Owned Subsidiaries	$ 209.7	$ 194.7