Share-based compensation - 2011 Director Plan (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based compensation
Income tax benefit	$ 0.7	$ 0.5	$ 0.9	$ 0.7
Common stock
Share-based compensation
Shares granted (in shares)	35,580	37,747	35,580	38,821
Fair value	$ 1.5	$ 1.3	$ 1.5	$ 1.3
Income tax benefit	$ 0.4	$ 0.3	$ 0.4	$ 0.3