Share-based compensation The 2011 Director Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Income tax benefit (in dollars)	$ 3.4	$ 2.8	$ 2.4
Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	33,170	33,184	29,448
Fair value of vested stock (in dollars)	$ 0.8	$ 0.8	$ 0.8
Income tax benefit (in dollars)	$ 0.3	$ 0.3	$ 0.3