Disclosures About Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Sep. 30, 2013	Mar. 31, 2013
Derivatives, Fair Value
Derivative Assets, Fair Value, Net Amount	$ 3,221	$ 43	$ 8,528
Derivative Liabilities, Net Amount	(372)	(6,677)	(29,765)
Interest rate contracts | Derivatives Designated as Hedging Instruments | Derivative financial instruments, assets
Derivatives, Fair Value
Derivative Assets, Fair Value, Net Amount	0		4,229
Interest rate contracts | Derivatives Designated as Hedging Instruments | Derivative financial instruments, liabilities
Derivatives, Fair Value
Derivative Liabilities, Net Amount	0		(24,298)
Commodity contracts | Derivatives Subject to Utility Rate Regulation | Derivative financial instruments, assets
Derivatives, Fair Value
Derivative Assets, Fair Value, Net Amount	3,153		4,130
Commodity contracts | Derivatives Subject to Utility Rate Regulation | Derivative financial instruments and other noncurrent liabilities
Derivatives, Fair Value
Derivative Liabilities, Net Amount	(372)		(5,467)
Commodity contracts | Derivatives Not Designated as Hedging Instruments | Derivative financial instruments, assets
Derivatives, Fair Value
Derivative Assets, Fair Value, Net Amount	68		169
Commodity contracts | Derivatives Not Designated as Hedging Instruments | Derivative financial instruments, liabilities
Derivatives, Fair Value
Derivative Liabilities, Net Amount	$ 0		$ 0