Liabilities - Other Liabilities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Other Assets
Other Liabilities
Cash surrender value of life insurance policies	$ 3.6	$ 3.4
Other Liabilities
Other Liabilities
Plan liabilities	3.9	4.7
Asset retirement obligations	3.2	3.3
Medical and dental benefits	$ 2.1	$ 2.2