Regulatory Assets and Liabilities and Regulatory Matters - Schedule of Regulatory Assets and Liabilities Associated With Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Regulatory Assets and Liabilities
Regulatory Assets		$ 301,819	$ 304,208	$ 270,599
Regulatory Liabilities	[1]	63,182	71,004	32,888
Postretirement Benefits
Regulatory Assets and Liabilities
Regulatory Liabilities		20,314	19,975	18,959
Deferred Fuel and Power Refunds
Regulatory Assets and Liabilities
Regulatory Liabilities		28,083	36,638	0
State Tax Benefits -- Distribution System Repairs
Regulatory Assets and Liabilities
Regulatory Liabilities		13,712	13,266	10,349
Other
Regulatory Assets and Liabilities
Regulatory Liabilities		1,073	1,125	3,580
Income Taxes Recoverable
Regulatory Assets and Liabilities
Regulatory Assets		117,396	115,946	111,075
Underfunded Pension and Postretirement Plans
Regulatory Assets and Liabilities
Regulatory Assets		138,294	140,762	107,827
Environmental Costs
Regulatory Assets and Liabilities
Regulatory Assets		17,643	19,983	14,738
Deferred Fuel and Power Costs
Regulatory Assets and Liabilities
Regulatory Assets		0	0	16,761
Removal Costs, Net
Regulatory Assets and Liabilities
Regulatory Assets		22,346	21,223	17,550
Other
Regulatory Assets and Liabilities
Regulatory Assets		$ 6,140	$ 6,294	$ 2,648

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