Retirement Plans & Other Postretirement Benefits - Fair Value of Guaranteed Annuity Contract (Details) - Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair value of guaranteed annuity contract [Abstract]
Fair value, beginning of year	$ 4.4	$ 4.3
Unrealized gains related to investments still held at reporting date	0.2	0.2
Purchases, sales & settlements, net	(0.1)	(0.1)
Fair value, end of year	$ 4.5	$ 4.4