Northern Trust Corporation (Corporation only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
ASSETS
Cash on Deposit with Subsidiary Bank	$ 3,056.8	$ 4,389.5
Other Assets	8,573.6	8,384.9
Total Assets	183,889.8	170,003.9	$ 136,828.4
LIABILITIES
Senior Notes	2,505.5	3,122.4
Floating Rate Capital Debt	0.0	277.8
Other Liabilities	4,177.5	5,536.6
Total Liabilities	171,873.0	158,315.6
STOCKHOLDERS' EQUITY
Common Stock	408.6	408.6
Additional Paid-In Capital	939.3	963.6
Retained Earnings	13,117.3	12,207.7
Accumulated Other Comprehensive Income (Loss)	(35.6)	428.0
Treasury Stock	(3,297.7)	(3,204.5)
Total Stockholders’ Equity	12,016.8	11,688.3	11,091.0	$ 10,508.3
Total Liabilities and Stockholders’ Equity	183,889.8	170,003.9
Parent Company
ASSETS
Cash on Deposit with Subsidiary Bank	1,731.7	2,516.0	$ 2,559.1	$ 866.8
Other Assets	890.2	900.9
Total Assets	16,738.1	17,059.6
LIABILITIES
Senior Notes	2,505.5	3,122.4
Long-Term Debt	1,145.7	1,189.3
Floating Rate Capital Debt	0.0	277.8
Other Liabilities	1,070.1	781.8
Total Liabilities	4,721.3	5,371.3
STOCKHOLDERS' EQUITY
Preferred Stock	884.9	884.9
Common Stock	408.6	408.6
Additional Paid-In Capital	939.3	963.6
Retained Earnings	13,117.3	12,207.7
Accumulated Other Comprehensive Income (Loss)	(35.6)	428.0
Treasury Stock	(3,297.7)	(3,204.5)
Total Stockholders’ Equity	12,016.8	11,688.3
Total Liabilities and Stockholders’ Equity	16,738.1	17,059.6
Parent Company | Banks Industry
ASSETS
Advances to Wholly-Owned Subsidiaries – Banks	2,810.0	2,670.0
Investments in Wholly-Owned Subsidiaries	11,120.9	10,799.9
Parent Company | Nonbank Subsidiaries
ASSETS
Investments in Wholly-Owned Subsidiaries	$ 185.3	$ 172.8