Share-based compensation Share-based compensation - The 2011 Director Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Income tax benefit (in dollars)	$ 1.6	$ 2.3	$ 3.4
Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	19,846	28,246	33,170
Fair value of vested stock (in dollars)	$ 0.6	$ 0.8	$ 0.8
Income tax benefit (in dollars)	$ 0.2	$ 0.3	$ 0.3