Postretirement Benefit Plans - Trust Assets Measured at Fair Value (Details) - Postretirement Benefit Plans [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets:
Total Assets	$ 64.4	$ 72.3	$ 92.4
Fair Value, Recurring [Member]
Assets:
Total Assets	64.4	72.3
Fair Value, Recurring [Member] | Cash and cash equivalents [Member]
Assets:
Total Assets	9.4	9.9
Fair Value, Recurring [Member] | Common Collective funds - domestic equities [Member]
Assets:
Total Assets	7.4	6.8
Fair Value, Recurring [Member] | Common collective funds - international equities [Member]
Assets:
Total Assets	4.2	5.2
Fair Value, Recurring [Member] | Fixed Income Funds [Member]
Assets:
Total Assets	$ 43.4	$ 50.4