Reporting Segments and Related Information - Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 3,852.1	$ 3,753.7	$ 3,434.3
Foreign Exchange Trading Income	250.9	307.2	209.9
Other Noninterest Income	292.2	276.6	301.9
Total Noninterest Income	4,395.2	4,337.5	3,946.1
Net Interest Income	1,710.7	1,663.9	1,475.0
Revenue	6,105.9	6,001.4	5,421.1
Provision for Credit Losses	(14.5)	(14.5)	(28.0)
Noninterest Expense	4,143.5	4,016.9	3,769.4
Income (loss) before Income Taxes	1,976.9	1,999.0	1,679.7
Provision (benefit) for Income Taxes	484.7	442.6	480.7
Net Income	1,492.2	1,556.4	1,199.0
Average Assets	117,551.4	122,946.6	119,607.4
Segment reporting information, fully taxable-equivalent adjustment	32.8	41.2	45.8
Corporate and Institutional Services Results of Operations
Noninterest Income
Trust, Investment and Other Servicing Fees	2,211.5	2,173.1	1,984.6
Foreign Exchange Trading Income	232.2	233.4	197.9
Other Noninterest Income	178.2	183.0	176.1
Total Noninterest Income	2,621.9	2,589.5	2,358.6
Net Interest Income	918.7	992.2	733.8
Revenue	3,540.6	3,581.7	3,092.4
Provision for Credit Losses	1.9	1.9	3.4
Noninterest Expense	2,605.5	2,421.4	2,194.5
Income (loss) before Income Taxes	933.2	1,158.4	894.5
Provision (benefit) for Income Taxes	219.4	255.3	279.5
Net Income	$ 713.8	$ 903.1	$ 615.0
Percentage of Consolidated Net Income	48.00%	58.00%	51.00%
Average Assets	$ 87,557.1	$ 82,996.5	$ 80,105.6
Wealth Management
Noninterest Income
Trust, Investment and Other Servicing Fees	1,640.6	1,580.6	1,449.7
Foreign Exchange Trading Income	18.7	4.2	3.1
Other Noninterest Income	131.1	102.7	103.9
Total Noninterest Income	1,790.4	1,687.5	1,556.7
Net Interest Income	792.0	816.5	736.2
Revenue	2,582.4	2,504.0	2,292.9
Provision for Credit Losses	(16.4)	(16.4)	(31.4)
Noninterest Expense	1,531.6	1,460.0	1,405.3
Income (loss) before Income Taxes	1,067.2	1,060.4	919.0
Provision (benefit) for Income Taxes	271.1	262.1	347.2
Net Income	$ 796.1	$ 798.3	$ 571.8
Percentage of Consolidated Net Income	53.00%	51.00%	48.00%
Average Assets	$ 29,994.3	$ 26,163.7	$ 26,599.9
Treasury and Other Results of Operations
Noninterest Income
Total Noninterest Income	(17.1)	60.5	30.8
Net Interest Income	0.0	(144.8)	5.0
Revenue	(17.1)	(84.3)	35.8
Noninterest Expense	6.4	135.5	169.6
Income (loss) before Income Taxes	(23.5)	(219.8)	(133.8)
Provision (benefit) for Income Taxes	(5.8)	(74.8)	(146.0)
Net Income	$ (17.7)	$ (145.0)	$ 12.2
Percentage of Consolidated Net Income	(1.00%)	(9.00%)	1.00%
Average Assets	$ 0.0	$ 13,786.4	$ 12,901.9