Regulatory Assets and Liabilities and Regulatory Matters - Regulatory Assets and Liabilities Associated with Gas Utility and Electric Utility (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Regulatory Assets and Liabilities
Regulatory assets	$ 268,166	$ 244,911
Regulatory liabilities	32,497	37,287
Postretirement Benefits
Regulatory Assets and Liabilities
Regulatory liabilities	18,594	16,497
Environmental Overcollections
Regulatory Assets and Liabilities
Regulatory liabilities	349	2,552
Deferred Fuel and Power Refunds
Regulatory Assets and Liabilities
Regulatory liabilities	306	8,283
State Tax Benefits — Distribution System Repairs
Regulatory Assets and Liabilities
Regulatory liabilities	10,076	8,453
Other
Regulatory Assets and Liabilities
Regulatory liabilities	3,172	1,502
Income Taxes Recoverable
Regulatory Assets and Liabilities
Regulatory assets	110,709	106,069
Underfunded Pension and Postretirement Plans
Regulatory Assets and Liabilities
Regulatory assets	110,116	94,515
Environmental Costs
Regulatory Assets and Liabilities
Regulatory assets	14,616	17,054
Deferred Fuel and Power Costs
Regulatory Assets and Liabilities
Regulatory assets	11,732	8,283
Removal Costs, Net
Regulatory Assets and Liabilities
Regulatory assets	16,790	13,333
Other
Regulatory Assets and Liabilities
Regulatory assets	$ 4,203	$ 5,657