Northern Trust Corporation (Corporation only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Cash on Deposit with Subsidiary Bank	$ 4,581.6	$ 4,518.1	$ 5,332.0	$ 6,418.5
Debt Securities	51,243.1	46,791.6
Other Assets	5,757.2	4,687.3
Total Assets	132,212.5	138,590.5	123,926.9
LIABILITIES
Senior Notes	2,011.3	1,497.3
Other Long-term Debt	1,112.4	1,449.5
Floating Rate Capital Debt	277.6	277.5
Other Liabilities	3,141.9	3,588.0
Total Liabilities	121,704.2	128,374.3
STOCKHOLDERS’ EQUITY
Common Stock	408.6	408.6
Additional Paid-In Capital	1,068.5	1,047.2
Retained Earnings	10,776.8	9,685.1
Accumulated Other Comprehensive Loss	(453.7)	(414.3)
Treasury Stock	(2,173.9)	(1,392.4)
Total Stockholders' Equity	10,508.3	10,216.2	9,770.4
Total Liabilities and Stockholders' Equity	132,212.5	138,590.5
Parent Company
ASSETS
Cash on Deposit with Subsidiary Bank	866.8	1,002.5	$ 757.0	$ 757.5
Debt Securities	0.0	0.9
Other Assets	803.1	706.4
Total Assets	14,348.0	13,620.1
LIABILITIES
Senior Notes	2,011.3	1,497.3
Other Long-term Debt	1,112.4	1,129.6
Floating Rate Capital Debt	277.6	277.5
Other Liabilities	438.5	499.5
Total Liabilities	3,839.8	3,403.9
STOCKHOLDERS’ EQUITY
Preferred Stock	882.0	882.0
Common Stock	408.6	408.6
Additional Paid-In Capital	1,068.4	1,047.2
Retained Earnings	10,776.8	9,685.1
Accumulated Other Comprehensive Loss	(453.7)	(414.3)
Treasury Stock	(2,173.9)	(1,392.4)
Total Stockholders' Equity	10,508.2	10,216.2
Total Liabilities and Stockholders' Equity	14,348.0	13,620.1
Parent Company | Banks Industry
ASSETS
Advances to Wholly-Owned Subsidiaries	2,910.0	2,460.0
Investments in Wholly-Owned Subsidiaries	9,585.2	9,223.9
Parent Company | Nonbank Subsidiaries
ASSETS
Advances to Wholly-Owned Subsidiaries	0.0	13.5
Investments in Wholly-Owned Subsidiaries	$ 182.9	$ 212.9