Background and Basis of Presentation - Marketable Securities (Details) $ in Millions	Mar. 31, 2018 USD ($)
Marketable securities
Cost of marketable securities	$ 4.1
Pension Benefits
Held to maturity
Investments restricted to the payment of post-retirement employee benefits	1.7
Other Assets | Pension Benefits
Held to maturity
Investments restricted to the payment of post-retirement employee benefits	1.1
Prepaid Expenses and Other Current Assets | Pension Benefits
Held to maturity
Investments restricted to the payment of post-retirement employee benefits	0.6
Fair Value | Level 1 | Other Assets
Marketable securities
Fair value of marketable securities	$ 3.6