Northern Trust Corporation (Corporation only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
ASSETS
Cash on Deposit with Subsidiary Bank	$ 4,654.2	$ 3,056.8
Other Assets	9,855.2	8,573.6
Total Assets	155,036.7	183,889.8	$ 170,003.9
LIABILITIES
Senior Notes	2,724.2	2,505.5
Other Liabilities	4,998.3	4,177.5
Total Liabilities	143,777.2	171,873.0
STOCKHOLDERS' EQUITY
Common Stock	408.6	408.6
Additional Paid-In Capital	983.5	939.3
Retained Earnings	13,798.5	13,117.3
Accumulated Other Comprehensive Loss	(1,569.2)	(35.6)
Treasury Stock	(3,246.8)	(3,297.7)
Total Stockholders’ Equity	11,259.5	12,016.8	11,688.3	$ 11,080.9
Total Liabilities and Stockholders’ Equity	155,036.7	183,889.8
Parent Company
ASSETS
Cash on Deposit with Subsidiary Bank	601.4	1,731.7	$ 2,516.0	$ 2,559.1
Other Assets	1,226.3	890.2
Total Assets	16,929.5	16,738.1
LIABILITIES
Senior Notes	2,724.2	2,505.5
Long-Term Debt	2,066.2	1,145.7
Other Liabilities	879.6	1,070.1
Total Liabilities	5,670.0	4,721.3
STOCKHOLDERS' EQUITY
Preferred Stock	884.9	884.9
Common Stock	408.6	408.6
Additional Paid-In Capital	983.5	939.3
Retained Earnings	13,798.5	13,117.3
Accumulated Other Comprehensive Loss	(1,569.2)	(35.6)
Treasury Stock	(3,246.8)	(3,297.7)
Total Stockholders’ Equity	11,259.5	12,016.8
Total Liabilities and Stockholders’ Equity	16,929.5	16,738.1
Parent Company | Banks Industry
ASSETS
Advances to Wholly-Owned Subsidiaries – Banks	4,010.0	2,810.0
Investments in Wholly-Owned Subsidiaries	10,897.1	11,120.9
Parent Company | Nonbank Subsidiaries
ASSETS
Investments in Wholly-Owned Subsidiaries	$ 194.7	$ 185.3