Regulatory Assets and Liabilities and Regulatory Matters - Regulatory Assets and Liabilities Associated with Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands		Sep. 30, 2015	Sep. 30, 2014
Regulatory Assets and Liabilities
Regulatory assets		$ 304,208	$ 268,166
Regulatory liabilities	[1]	71,004	32,497
Postretirement Benefits
Regulatory Assets and Liabilities
Regulatory liabilities	[1]	19,975	18,594
Environmental Overcollections
Regulatory Assets and Liabilities
Regulatory liabilities	[1]	0	349
Deferred Fuel and Power Refunds
Regulatory Assets and Liabilities
Regulatory liabilities	[1]	36,638	306
State Tax Benefits — Distribution System Repairs
Regulatory Assets and Liabilities
Regulatory liabilities	[1]	13,266	10,076
Other
Regulatory Assets and Liabilities
Regulatory liabilities	[1]	1,125	3,172
Income Taxes Recoverable
Regulatory Assets and Liabilities
Regulatory assets		115,946	110,709
Underfunded Pension and Postretirement Plans
Regulatory Assets and Liabilities
Regulatory assets		140,762	110,116
Environmental Costs
Regulatory Assets and Liabilities
Regulatory assets		19,983	14,616
Deferred Fuel and Power Costs
Regulatory Assets and Liabilities
Regulatory assets		0	11,732
Removal Costs, Net
Regulatory Assets and Liabilities
Regulatory assets		21,223	16,790
Other
Regulatory Assets and Liabilities
Regulatory assets		$ 6,294	$ 4,203

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