Accumulated Other Comprehensive Income Affected Line Item in the Statement of Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance	$ (124,675)	$ (111,086)	$ (240,432)	$ (217,613)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	216,963	178,559	353,436	303,657
Income Tax Expense	(83,596)	(66,219)	(133,053)	(113,969)
Income (loss) from continuing operations	133,367	112,340	220,383	189,688
Interest charges	(31,601)	(33,331)	(63,716)	(63,853)
Purchased gas cost	(1,468,045)	(876,245)	(2,334,236)	(1,548,038)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations	3,938	(2,018)	1,674	(5,485)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance	358	2,689	358	2,689
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	358	2,689	358	2,689
Income Tax Expense	(131)	(981)	(131)	(981)
Income (loss) from continuing operations	227	1,708	227	1,708
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Interest Rate Hedges [Member] [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(1,057)	(873)	(2,115)	(1,375)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Commodity Contracts Cash Flow Hedges [Member] [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Purchased gas cost	7,184	(5,201)	4,574	(10,361)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	6,127	(6,074)	2,459	(11,736)
Income Tax Expense	(2,416)	2,348	(1,012)	4,543
Income (loss) from continuing operations	$ 3,711	$ (3,726)	$ 1,447	$ (7,193)