Reporting Segments and Related Information - Earnings Contribution of Northern Trust's Reporting Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Noninterest Income
Trust, Investment and Other Servicing Fees	$ 3,753.7	$ 3,434.3	$ 3,108.1
Foreign Exchange Trading Income	307.2	209.9	236.6
Other Noninterest Income	276.6	301.9	382.2
Net Interest Income (Note)	1,663.9	1,475.0	1,260.0
Noninterest Income	4,337.5	3,946.1	3,726.9
Revenue (Note)	6,001.4	5,421.1	4,986.9
Provision for Credit Losses	(14.5)	(28.0)	(26.0)
Noninterest Expense	4,016.9	3,769.4	3,470.7
Income before Income Taxes (Note)	1,999.0	1,679.7	1,542.2
Provision for Income Taxes (Note)	442.6	480.7	509.7
Net Income	1,556.4	1,199.0	1,032.5
Average Assets	122,946.6	119,607.4	115,570.3
Fully taxable equivalent basis adjustments	41.2	45.8	25.1
CORPORATE & INSTITUTIONAL SERVICES
Noninterest Income
Trust, Investment and Other Servicing Fees	2,173.1	1,984.6	1,787.8
Foreign Exchange Trading Income	233.4	197.9	224.4
Other Noninterest Income	183.0	176.1	147.0
Net Interest Income (Note)	992.2	733.8	565.0
Revenue (Note)	3,581.7	3,092.4	2,724.2
Provision for Credit Losses	1.9	3.4	1.9
Noninterest Expense	2,421.4	2,194.5	2,012.2
Income before Income Taxes (Note)	1,158.4	894.5	710.1
Provision for Income Taxes (Note)	255.3	279.5	212.9
Net Income	$ 903.1	$ 615.0	$ 497.2
Percentage of Consolidated Net Income	58.00%	51.00%	48.00%
Average Assets	$ 82,996.5	$ 80,105.6	$ 76,194.7
WEALTH MANAGEMENT
Noninterest Income
Trust, Investment and Other Servicing Fees	1,580.6	1,449.7	1,320.3
Foreign Exchange Trading Income	4.2	3.1	8.6
Other Noninterest Income	102.7	103.9	105.7
Net Interest Income (Note)	816.5	736.2	651.4
Revenue (Note)	2,504.0	2,292.9	2,086.0
Provision for Credit Losses	(16.4)	(31.4)	(27.9)
Noninterest Expense	1,460.0	1,405.3	1,315.3
Income before Income Taxes (Note)	1,060.4	919.0	798.6
Provision for Income Taxes (Note)	262.1	347.2	301.1
Net Income	$ 798.3	$ 571.8	$ 497.5
Percentage of Consolidated Net Income	51.00%	48.00%	48.00%
Average Assets	$ 26,163.7	$ 26,599.9	$ 26,525.0
Treasury and Other
Noninterest Income
Net Interest Income (Note)	(144.8)	5.0	43.6
Noninterest Income	60.5	30.8	133.1
Revenue (Note)	(84.3)	35.8	176.7
Noninterest Expense	135.5	169.6	143.2
Income before Income Taxes (Note)	(219.8)	(133.8)	33.5
Provision for Income Taxes (Note)	(74.8)	(146.0)	(4.3)
Net Income	$ (145.0)	$ 12.2	$ 37.8
Percentage of Consolidated Net Income	(9.00%)	1.00%	4.00%
Average Assets	$ 13,786.4	$ 12,901.9	$ 12,850.6