Segment Information - Revenue from External Customers by Products and Services (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	$ 3,115,546	$ 2,759,735	$ 2,454,648
Distribution
Revenues from External Customers and Long-Lived Assets [Line Items]
Other gas revenues	8,999	31,204	24,940
Total distribution revenues	3,000,404	2,647,813	2,338,404
Distribution | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	2,892,155	2,530,277	2,236,774
Distribution | Natural Gas, Gathering, Transportation, Marketing and Processing
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	99,250	86,332	76,690
Distribution | Residential | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	1,916,101	1,642,918	1,477,049
Distribution | Commercial | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	797,073	708,167	619,979
Distribution | Industrial | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	131,267	133,372	98,439
Distribution | Public authority and other | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	47,714	45,820	41,307
Pipeline and Storage
Revenues from External Customers and Long-Lived Assets [Line Items]
Pipeline and storage revenues	$ 115,142	$ 111,922	$ 116,244