Share-based Compensation - Share and performance-based compensation expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share and performance-based compensation expense [Abstract]
Performance-based share expense	$ (0.1)	$ 4.0	$ 2.4
Service-based share expense	3.8	3.4	2.1
Total share-based compensation expense before income tax	3.7	7.4	4.5
Income tax	0.0	0.0	0.0
Total share-based compensation expense, net of income tax	3.7	$ 7.4	$ 4.5
Unrecognized compensation expense	$ 6.6
Weighted average period of expense recognition	2 years