Northern Trust Corporation (Corporation only) - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES:
Net Income	$ 1,199.0	$ 1,032.5	$ 973.8
Adjustments to Reconcile Net Income to Net Cash Provided by (Used in) Operating Activities:
Change in Accrued Income Taxes	36.2	(129.0)	206.8
Other, net	(302.1)	653.4	(318.1)
Net Cash Provided by Operating Activities	1,720.4	1,510.0	1,834.4
INVESTING ACTIVITIES:
Proceeds from Sale, Maturity and Redemption of Securities – Available for Sale	10,103.4	11,317.3	8,576.1
Other, net	25.8	226.5	(212.9)
Net Cash Used in Investing Activities	(14,021.6)	(10,192.1)	(6,900.1)
FINANCING ACTIVITIES:
Proceeds from Issuance of Preferred Stock - Series D	0.0	493.5	0.0
Treasury Stock Purchased	(523.1)	(411.1)	(496.9)
Net Proceeds from Stock Options	108.0	233.8	94.0
Cash Dividends Paid on Common Stock	(356.8)	(333.0)	(321.4)
Cash Dividends Paid on Preferred Stock	(49.8)	(23.4)	(27.0)
Other, net	0.1	(7.5)	17.8
Net Cash Provided by Financing Activities	11,252.7	7,536.9	8,534.1
(Decrease) Increase in Cash and Due from Banks	(813.9)	(1,086.5)	3,397.5
Cash and Due from Banks at Beginning of Year	5,332.0	6,418.5	3,021.0
Cash and Due from Banks at End of Year	4,518.1	5,332.0	6,418.5
Parent Company
OPERATING ACTIVITIES:
Net Income	1,199.0	1,032.5	973.8
Adjustments to Reconcile Net Income to Net Cash Provided by (Used in) Operating Activities:
Equity in Undistributed Net Income of Subsidiaries	(656.4)	(736.9)	(408.3)
Change in Prepaid Expenses	(0.3)	3.0	1.2
Change in Accrued Income Taxes	17.2	(17.9)	22.8
Other, net	55.7	55.7	58.4
Net Cash Provided by Operating Activities	615.2	336.4	647.9
INVESTING ACTIVITIES:
Proceeds from Sale, Maturity and Redemption of Securities – Available for Sale	0.0	0.2	1.3
Change in Capital Investments in Subsidiaries	0.0	(3.0)	(10.0)
Advances to Wholly-Owned Subsidiaries	100.0	(295.0)	0.0
Other, net	1.9	1.2	0.2
Net Cash Used in Investing Activities	101.9	(296.6)	(8.5)
FINANCING ACTIVITIES:
Proceeds from Senior Notes and Long-Term Debt	350.0	0.0	0.0
Proceeds from Issuance of Preferred Stock - Series D	0.0	493.5	0.0
Treasury Stock Purchased	(523.1)	(411.1)	(496.9)
Net Proceeds from Stock Options	108.0	233.8	94.0
Cash Dividends Paid on Common Stock	(356.8)	(333.0)	(321.4)
Cash Dividends Paid on Preferred Stock	(49.8)	(23.4)	(27.0)
Other, net	0.1	(0.1)	0.0
Net Cash Provided by Financing Activities	(471.6)	(40.3)	(751.3)
(Decrease) Increase in Cash and Due from Banks	245.5	(0.5)	(111.9)
Cash and Due from Banks at Beginning of Year	757.0	757.5	869.4
Cash and Due from Banks at End of Year	$ 1,002.5	$ 757.0	$ 757.5