Pension and Other Post-Retirement Benefits - Fair Value Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair value of pension plan assets
Assets	$ 435.6	$ 413.5
Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Assets	2.6	3.1
Fair Value, Inputs, Level 2 [Member]
Fair value of pension plan assets
Assets	0.0	0.0
Short-term investment funds [Member]
Fair value of pension plan assets
Assets	2.6	3.1
Short-term investment funds [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Assets	2.6	3.1
Short-term investment funds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair value of pension plan assets
Assets	0.0	0.0
Common stock collective funds [Member]
Fair value of pension plan assets
Assets	204.1
Fixed income collective trust funds [Member]
Fair value of pension plan assets
Assets	211.0	151.4
Real estate investment funds [Member]
Fair value of pension plan assets
Assets	14.8	18.7
Loan Fund [Member]
Fair value of pension plan assets
Assets	$ 3.1	29.9
UNITED STATES | Common stock [Member]
Fair value of pension plan assets
Assets		210.4
UNITED STATES | Common stock [Member] | Fair Value, Inputs, Level 1 [Member]
Fair value of pension plan assets
Assets