Fair Value Measurements (Details) - USD ($)	Dec. 31, 2016	Sep. 30, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	$ (94,053,000)	$ (32,515,000)
Derivative Asset	14,970,000	11,626,000
Hedged portion of gas stored underground	76,735,000	52,578,000
Available-for-sale Securities	82,130,000	72,701,000
Total assets	173,835,000	136,905,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(107,750,000)	(82,865,000)
Derivative Liability	122,995,000	240,819,000
Derivative, Cash Collateral	13,700,000	50,400,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	3,788,000	6,775,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	9,909,000	43,575,000
Additional Fair Value Elements [Abstract]
Debt Instrument Carrying Amount	2,585,000,000	2,460,000,000
Debt Instrument Fair Value	2,788,228,000	2,844,990,000
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	38,836,000	38,677,000
Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	10,378,000
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	31,303,000	31,394,000
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,613,000	2,630,000
Fair Value Inputs Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	0	0
Hedged portion of gas stored underground	76,735,000	52,578,000
Available-for-sale Securities	49,214,000	38,677,000
Total assets	125,949,000	91,255,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	0	0
Fair Value Inputs Level 1 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	38,836,000	38,677,000
Fair Value Inputs Level 1 [Member] | Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	10,378,000
Fair Value Inputs Level 1 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 1 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	109,023,000	44,141,000
Hedged portion of gas stored underground	0	0
Available-for-sale Securities	32,916,000	34,024,000
Total assets	141,939,000	78,165,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	230,745,000	323,684,000
Fair Value Inputs Level 2 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 2 [Member] | Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0
Fair Value Inputs Level 2 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	31,303,000	31,394,000
Fair Value Inputs Level 2 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,613,000	2,630,000
Fair Value Inputs Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	0	0
Hedged portion of gas stored underground	0	0
Available-for-sale Securities	0	0
Total assets	0	0
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	0	0
Fair Value Inputs Level 3 [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 3 [Member] | Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0
Fair Value Inputs Level 3 [Member] | Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0
Fair Value Inputs Level 3 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 0	$ 0