Retirement Plans & Other Postretirement Benefits - Prior Service Cost & Actuarial Gains and Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Prior service costs [Roll Forward]
Balance at beginning of period	$ 1.4	$ 1.8	$ 2.0
Amounts arising during the period	1.3	0.0	0.5
Reclassification to benefit costs	(0.4)	(0.4)	(0.7)
Balance at end of period	2.3	1.4	1.8
Net gain or loss [Roll Forward]
Balance at beginning of period	114.6	110.1	111.7
Amounts arising during the period	3.1	11.7	6.9
Reclassification to benefit costs	(7.4)	(7.2)	(8.5)
Balance at end of period	110.3	114.6	110.1
Other Benefits
Prior service costs [Roll Forward]
Balance at beginning of period	(11.2)	(14.1)	(17.1)
Amounts arising during the period	0.0	0.0	0.0
Reclassification to benefit costs	2.4	2.9	3.0
Balance at end of period	(8.8)	(11.2)	(14.1)
Net gain or loss [Roll Forward]
Balance at beginning of period	0.0	1.6	10.9
Amounts arising during the period	1.2	(1.6)	(8.6)
Reclassification to benefit costs	0.0	0.0	(0.7)
Balance at end of period	$ 1.2	$ 0.0	$ 1.6