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Share-based compensation The 2011 Director Plan (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Income tax benefit (in dollars) $ 2.3 $ 3.4 $ 2.8
Common stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted 28,246 33,170 33,184
Fair value of vested stock (in dollars) $ 0.8 $ 0.8 $ 0.8
Income tax benefit (in dollars) $ 0.3 $ 0.3 $ 0.3