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PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS Part 3 (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2011
Pension Plans [Member]
      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of year $ 412,171 $ 384,163  
Service cost 9,209 9,203 9,553
Interest cost 16,959 19,358 18,819
Actuarial (gain) loss (23,921) 52,161  
MGE acquisition 151,424 0  
Settlement loss 24,999 14,348  
Gross benefits paid (87,023) [1] (67,062) [1]  
Benefit obligation, end of year 503,818 412,171 384,163
Accumulated benefit obligation, end of year 444,129 353,061  
Postretirement Plans [Member]
      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of year 127,217 103,991  
Service cost 10,162 8,060 7,676
Interest cost 5,234 5,521 4,843
Actuarial (gain) loss (17,514) (15,895)  
MGE acquisition 28,444 0  
Gross benefits paid (8,449) (6,250)  
Benefit obligation, end of year $ 180,122 $ 127,217 $ 103,991
[1] Includes $79,484 and $60,085 lump-sum payments recognized as settlements in fiscal years 2013 and 2012, respectively.