Changes in Level 3 U.S. Pension Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
U.S. Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	$ 1,342.0	$ 1,440.8
Actual Return on Plan Assets	110.0	(25.0)
Purchases	0.0	0.0
Fair Value at end of year	1,393.5	1,342.0
U.S. Plan | Fair Value, Inputs, 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	47.5	49.0
Actual Return on Plan Assets	(5.6)	6.6
Purchases	2.0	2.4
Sales	(8.2)	(10.5)
Fair Value at end of year	35.7	47.5
U.S. Plan | Fair Value, Inputs, Level 3 | Northern Trust Hedge Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	63.4	68.6
Actual Return on Plan Assets	1.5	(0.2)
Realized Gain	0.0	2.6
Purchases	0.0	10.0
Sales	(0.1)	(17.6)
Fair Value at end of year	64.8	63.4
Non U.S. Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value at beginning of year	144.3	157.6
Actual Return on Plan Assets	21.9	1.3
Purchases	4.3	4.6
Fair Value at end of year	$ 139.3	$ 144.3