Regulatory Assets and Liabilities and Regulatory Matters - Schedule of Regulatory Assets and Liabilities Associated With Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016
Regulatory Assets
Regulatory assets	$ 398,747	$ 395,141	$ 345,300
Regulatory Liabilities
Regulatory liabilities	48,670	55,569	69,856
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities	16,715	17,519	19,671
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities	12,587	22,299	34,432
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities	16,662	15,086	14,604
Other
Regulatory Liabilities
Regulatory liabilities	2,706	665	1,149
Income taxes recoverable
Regulatory Assets
Regulatory assets	122,733	115,643	119,604
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets	171,833	183,129	133,356
Environmental costs
Regulatory Assets
Regulatory assets	61,616	59,397	60,716
Deferred fuel and power costs
Regulatory Assets
Regulatory assets	7,024	151	0
Removal costs, net
Regulatory Assets
Regulatory assets	29,405	27,956	22,444
Other
Regulatory Assets
Regulatory assets	$ 6,136	$ 8,865	$ 9,180