Debt - Maturities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Annual maturities of debt:
2019	$ 15.0
2020	194.9
2022	104.9
Thereafter	100.8
Total debt	415.6	$ 411.7
Interest expense	$ 15.4	$ 13.2	$ 13.0