Segment Information - Revenue from External Customers by Products and Services (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	$ 2,901,848	$ 3,115,546	$ 2,759,735
Distribution
Revenues from External Customers and Long-Lived Assets [Line Items]
Other gas revenues	41,704	8,999	31,204
Total distribution revenues	2,742,824	3,000,404	2,647,813
Distribution | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	2,605,491	2,892,155	2,530,277
Distribution | Natural Gas, Gathering, Transportation, Marketing and Processing
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	95,629	99,250	86,332
Distribution | Residential | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	1,733,548	1,916,101	1,642,918
Distribution | Commercial | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	711,284	797,073	708,167
Distribution | Industrial | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	118,046	131,267	133,372
Distribution | Public authority and other | Natural Gas, US Regulated
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	42,613	47,714	45,820
Pipeline and Storage
Revenues from External Customers and Long-Lived Assets [Line Items]
Pipeline and storage revenues	$ 159,024	$ 115,142	$ 111,922