Employee Benefits - Changes in Level 3 Assets (Detail) - Pension Plans, Defined Benefit - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	$ 1,506.4
Fair Value at end of year	1,380.1	$ 1,506.4
Northern Trust Private Equity Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	29.3
Fair Value at end of year	25.5	29.3
Northern Trust Hedge Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	44.6
Fair Value at end of year	29.2	44.6
LEVEL 3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	73.9
Fair Value at end of year	54.7	73.9
LEVEL 3 | Northern Trust Private Equity Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	29.3	35.7
Actual Return on Plan Assets	(1.5)	(5.4)
Realized Gain	0.0	0.0
Purchases	0.3	0.8
Sales	(2.6)	(1.8)
Fair Value at end of year	25.5	29.3
LEVEL 3 | Northern Trust Hedge Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	44.6	64.8
Actual Return on Plan Assets	(2.7)	(3.1)
Realized Gain	2.4	5.0
Purchases	0.0	0.0
Sales	(15.1)	(22.1)
Fair Value at end of year	$ 29.2	$ 44.6