Retirement Benefits - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 0.1
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year		$ 0.6
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year		$ 1.1