Share-based compensation - 2011 Director Plan (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share-based compensation
Income tax benefit from compensation expense	$ 0.8	$ 0.4	$ 1.2	$ 0.7
Common stock
Share-based compensation
Shares granted (in shares)	35,000	0	35,770	0
Fair value measurement of shares granted and vested	$ 1.1	$ 0.0	$ 1.2	$ 0.0
Income tax benefit from compensation expense	$ 0.4	$ 0.0	$ 0.5	$ 0.0