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Nonqualified defined benefit plan (Details 11) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit and Defined Contribution Plans Disclosure [Line Items]      
Investments in nonqualified benefit plans $ 105.2 $ 101.4  
Supplemental employee retirement plan, defined benefit [Member]      
Defined Benefit and Defined Contribution Plans Disclosure [Line Items]      
Net periodic benefit cost 7.1 6.6 $ 7.3
Projected benefit obligation 110.8 115.6  
Accumulated benefit obligation $ 104.6 $ 108.2  
Discount rate to determine benefit obligation 3.77% 3.51%  
Rate of compensation increase to determine benefit obligation 4.00% 4.00%  
Discount rate to determine net periodic benefit cost 3.51% 4.32%  
Rate of compensation increase to determine net periodic benefit cost 4.00% 4.00%  
Expected future benefit payments, next twelve months $ 6.5    
Expected future benefit payments, year two 6.7    
Expected future benefit payments, year three 7.1    
Expected future benefit payments, year four 7.3    
Expected future benefit payments, year five 7.8    
Expected future benefit payments, five fiscal years thereafter $ 37.7