Regulatory Assets and Liabilities and Regulatory Matters - Schedule of Regulatory Assets and Liabilities Associated With Gas Utility and Electric Utility (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Sep. 30, 2014		Mar. 31, 2014
Regulatory Assets and Liabilities
Regulatory Assets	$ 252,570		$ 268,166		$ 235,931
Regulatory Liabilities	72,585	[1]	32,497	[1]	33,148	[1]
Postretirement Benefits
Regulatory Assets and Liabilities
Regulatory Liabilities	19,323		18,594		17,196
Environmental Overcollections
Regulatory Assets and Liabilities
Regulatory Liabilities	0		349		1,855
Deferred Fuel and Power Refunds
Regulatory Assets and Liabilities
Regulatory Liabilities	40,542		306		3,204
State Tax Benefits -- Distribution System Repairs
Regulatory Assets and Liabilities
Regulatory Liabilities	10,621		10,076		8,998
Other
Regulatory Assets and Liabilities
Regulatory Liabilities	2,099		3,172		1,895
Income Taxes Recoverable
Regulatory Assets and Liabilities
Regulatory Assets	111,441		110,709		106,800
Underfunded Pension and Postretirement Plans
Regulatory Assets and Liabilities
Regulatory Assets	105,539		110,116		90,996
Environmental Costs
Regulatory Assets and Liabilities
Regulatory Assets	14,110		14,616		14,544
Deferred Fuel and Power Costs
Regulatory Assets and Liabilities
Regulatory Assets	0		11,732		4,308
Removal Costs, Net
Regulatory Assets and Liabilities
Regulatory Assets	18,377		16,790		14,379
Other
Regulatory Assets and Liabilities
Regulatory Assets	$ 3,103		$ 4,203		$ 4,904

[1] Regulatory liabilities, other than deferred fuel and power refunds, are recorded in other current and noncurrent liabilities in the Condensed Consolidated Balance Sheets.