Northern Trust Corporation (Corporation only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ASSETS
Cash on Deposit with Subsidiary Bank	$ 4,389.5	$ 4,459.2
Other Assets	8,384.9	8,401.3
Total Assets	170,003.9	136,828.4	$ 132,212.5
LIABILITIES
Senior Notes	3,122.4	2,573.0
Floating Rate Capital Debt	277.8	277.7
Other Liabilities	5,536.6	4,830.6
Total Liabilities	158,315.6	125,737.4
STOCKHOLDERS' EQUITY
Common Stock	408.6	408.6
Additional Paid-In Capital	963.6	1,013.1
Retained Earnings	12,207.7	11,656.7
Accumulated Other Comprehensive Income (Loss)	428.0	(194.7)
Treasury Stock	(3,204.5)	(3,066.1)
Total Stockholders’ Equity	11,688.3	11,091.0	10,508.3	$ 10,216.2
Total Liabilities and Stockholders’ Equity	170,003.9	136,828.4
Parent Company
ASSETS
Cash on Deposit with Subsidiary Bank	2,516.0	2,559.1	$ 866.8	$ 1,002.5
Other Assets	900.9	1,444.7
Total Assets	17,059.6	15,886.6
LIABILITIES
Senior Notes	3,122.4	2,573.0
Long-Term Debt	1,189.3	1,148.1
Floating Rate Capital Debt	277.8	277.7
Other Liabilities	781.8	796.8
Total Liabilities	5,371.3	4,795.6
STOCKHOLDERS' EQUITY
Preferred Stock	884.9	1,273.4
Common Stock	408.6	408.6
Additional Paid-In Capital	963.6	1,013.1
Retained Earnings	12,207.7	11,656.7
Accumulated Other Comprehensive Income (Loss)	428.0	(194.7)
Treasury Stock	(3,204.5)	(3,066.1)
Total Stockholders’ Equity	11,688.3	11,091.0
Total Liabilities and Stockholders’ Equity	17,059.6	15,886.6
Parent Company | Banks Industry
ASSETS
Advances to Wholly-Owned Subsidiaries	2,670.0	2,370.0
Investments in Wholly-Owned Subsidiaries	10,799.9	9,349.8
Parent Company | Nonbank Subsidiaries
ASSETS
Investments in Wholly-Owned Subsidiaries	$ 172.8	$ 163.0