Fair Value Measurements (Details) - USD ($)	Jun. 30, 2017	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	$ 0	$ (32,515,000)
Derivative Asset	3,228,000	11,626,000
Hedged portion of gas stored underground		52,578,000
Available-for-sale Securities	89,611,000	72,701,000
Total assets	92,839,000	136,905,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	0	(82,865,000)
Derivative Liability	109,372,000	240,819,000
Derivative, Cash Collateral		50,400,000
Cash Held In Margin Accounts Classified Current Risk Management Asset	0	6,775,000
Cash Held In Margin Accounts Offset Current Risk Management Liabilities	0	43,575,000
Additional Fair Value Elements [Abstract]
Debt Instrument Carrying Amount	3,085,000,000	2,460,000,000
Debt Instrument Fair Value	3,388,003,000	2,844,990,000
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	39,406,000	38,677,000
Fixed Income Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	15,892,000
Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	31,429,000	31,394,000
Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,884,000	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		52,578,000
Available-for-sale Securities	55,298,000	38,677,000
Total assets	55,298,000	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	39,406,000	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	15,892,000
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	3,228,000	44,141,000
Hedged portion of gas stored underground		0
Available-for-sale Securities	34,313,000	34,024,000
Total assets	37,541,000	78,165,000
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	109,372,000	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	31,429,000	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	2,884,000	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
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