Employee Benefits - Changes in Level 3 Assets (Detail) - U.S. Plan - Fair Value, Inputs, Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Northern Trust Private Equity Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	$ 35.7	$ 47.5
Actual Return on Plan Assets	(5.4)	(5.6)
Purchases	0.8	2.0
Sales	(1.8)	(8.2)
Fair Value at end of year	29.3	35.7
Northern Trust Hedge Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	64.8	63.4
Actual Return on Plan Assets	(3.1)	1.5
Realized Gain	5.0	0.0
Purchases	0.0	0.0
Sales	(22.1)	(0.1)
Fair Value at end of year	$ 44.6	$ 64.8