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Financial Instruments and Fair Value Measurement (Details) (USD $)
3 Months Ended 6 Months Ended
Dec. 31, 2012
item
Mar. 31, 2013
Sep. 30, 2012
Mar. 31, 2013
Equity securities
Mar. 31, 2012
Equity securities
Sep. 30, 2012
Equity securities
Available-for-sale securities            
Cost       $ 129,183,000   $ 129,183,000
Gross unrealized gains       363,593,000   304,396,000
Estimated fair value       492,776,000   433,579,000
Number of limited partnerships that entity sold shares of the partnership 3          
Sales proceeds of marketable equity available-for-sale securities       0 0  
Limited Liability Partnerships            
Realized gain from sale of investment securities 8,800,000          
Non-qualified Supplemental Savings Plan            
Assets held in Non-qualified Supplement Savings Plan, at Fair Value   $ 8,600,000 $ 8,200,000