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, 2016	Sep. 30, 2016	Dec. 31, 2015
Regulatory Assets
Regulatory assets		$ 392,864	$ 395,141	$ 301,819
Regulatory Liabilities
Regulatory liabilities	[1]	58,668	55,569	63,182
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities		17,259	17,519	20,314
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities		23,809	22,299	28,083
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities		15,579	15,086	13,712
Other
Regulatory Liabilities
Regulatory liabilities		2,021	665	1,073
Income taxes recoverable
Regulatory Assets
Regulatory assets		117,777	115,643	117,396
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets		179,364	183,129	138,294
Environmental costs
Regulatory Assets
Regulatory assets		61,437	59,397	17,643
Removal costs, net
Regulatory Assets
Regulatory assets		27,062	27,956	22,346
Other
Regulatory Assets
Regulatory assets		$ 7,224	$ 9,016	$ 6,140

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