Northern Trust Corporation (Corporation only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Cash on Deposit with Subsidiary Bank	$ 4,459.2	$ 4,581.6
Other Assets	8,401.3	5,757.2
Total Assets	136,828.4	132,212.5	$ 138,590.5
LIABILITIES
Senior Notes	2,573.0	2,011.3
Floating Rate Capital Debt	277.7	277.6
Other Liabilities	4,830.6	3,141.9
Total Liabilities	125,737.4	121,704.2
STOCKHOLDERS’ EQUITY
Common Stock	408.6	408.6
Additional Paid-In Capital	1,013.1	1,068.5
Retained Earnings	11,656.7	10,776.8
Accumulated Other Comprehensive Loss	(194.7)	(453.7)
Treasury Stock	(3,066.1)	(2,173.9)
Total Stockholders’ Equity	11,091.0	10,508.3	10,216.2	$ 9,770.4
Total Liabilities and Stockholders’ Equity	136,828.4	132,212.5
Parent Company
ASSETS
Cash on Deposit with Subsidiary Bank	2,559.1	866.8	$ 1,002.5	$ 757.0
Other Assets	1,444.7	803.1
Total Assets	15,886.6	14,348.0
LIABILITIES
Senior Notes	2,573.0	2,011.3
Long-Term Debt	1,148.1	1,112.4
Floating Rate Capital Debt	277.7	277.6
Other Liabilities	796.8	438.5
Total Liabilities	4,795.6	3,839.8
STOCKHOLDERS’ EQUITY
Preferred Stock	1,273.4	882.0
Common Stock	408.6	408.6
Additional Paid-In Capital	1,013.1	1,068.4
Retained Earnings	11,656.7	10,776.8
Accumulated Other Comprehensive Loss	(194.7)	(453.7)
Treasury Stock	(3,066.1)	(2,173.9)
Total Stockholders’ Equity	11,091.0	10,508.2
Total Liabilities and Stockholders’ Equity	15,886.6	14,348.0
Parent Company | Banks Industry
ASSETS
Advances to Wholly-Owned Subsidiaries	2,370.0	2,910.0
Investments in Wholly-Owned Subsidiaries	9,349.8	9,585.2
Parent Company | Nonbank Subsidiaries
ASSETS
Investments in Wholly-Owned Subsidiaries	$ 163.0	$ 182.9