Share-based compensation - 2011 Director Plan (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based compensation
Income tax benefit from compensation expense	$ 0.4	$ 0.5	$ 1.5	$ 1.2
Common stock
Share-based compensation
Shares granted (in shares)	0	19,846	35,770	19,846
Fair value measurement of shares granted and vested	$ 0.0	$ 0.6	$ 1.2	$ 0.6
Income tax benefit from compensation expense	$ 0.0	$ 0.2	$ 0.5	$ 0.2