Regulatory Assets and Liabilities and Regulatory Matters - Schedule of Regulatory Assets and Liabilities Associated With Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands		Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Regulatory Assets
Regulatory assets		$ 394,717	$ 395,141	$ 348,212
Regulatory Liabilities
Regulatory liabilities	[1]	50,458	55,569	66,803
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities		16,974	17,519	19,307
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities		13,791	22,299	30,838
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities		16,145	15,086	14,158
Other
Regulatory Liabilities
Regulatory liabilities		3,548	665	2,500
Income taxes recoverable
Regulatory Assets
Regulatory assets		120,255	115,643	118,160
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets		175,598	183,129	135,825
Environmental costs
Regulatory Assets
Regulatory assets		62,209	59,397	60,494
Deferred fuel and power costs
Regulatory Assets
Regulatory assets		1,262	151	0
Removal costs, net
Regulatory Assets
Regulatory assets		28,840	27,956	25,030
Other
Regulatory Assets
Regulatory assets		$ 6,553	$ 8,865	$ 8,703

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