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Pension Plan and Retirement Benefits - Investment Strategy (Details) - U.S. pension plan
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Asset allocation      
Plan's return on assets assumption (as a percent) 7.50% 7.50% 7.50%
Minimum      
Asset allocation      
Equity risk premium above broad bond market yields (as a percent) 3.00%    
Broad bond market yields (as a percent) 5.00%    
Maximum      
Asset allocation      
Equity risk premium above broad bond market yields (as a percent) 4.00%    
Broad bond market yields (as a percent) 7.00%    
Amount of adjustment to target allocation (as a percent) 10.00%    
Equity      
Asset allocation      
Target plan asset allocation (as a percent) 60.00%    
Fixed Income      
Asset allocation      
Target plan asset allocation (as a percent) 40.00%