Revenue from Contracts with Customers (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	$ 499,597
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	(528)
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	(8,822)
Revenue	490,247	$ 419,655
Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	174,823
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	(11,947)
Revenue	162,876
Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	77,069
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0
Revenue	77,069
Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	29,690
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0
Revenue	29,690
Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	223,185
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	(528)
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0
Revenue	222,657
Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	49,287
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	3,125
Revenue	52,412
All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,007
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0
Revenue	1,007
Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(55,464)
Alternative Revenue Programs Outside Scope of Authoritative Guidance on Revenue Recognition	0
Derivative Financial Instruments Outside Scope of Authoritative Guidance on Revenue Recognition	0
Revenue	(55,464)
Production of Natural Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	135,911
Production of Natural Gas [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	135,911
Production of Natural Gas [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Natural Gas [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Natural Gas [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Natural Gas [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Natural Gas [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Natural Gas [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Crude Oil [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	37,555
Production of Crude Oil [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	37,555
Production of Crude Oil [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Crude Oil [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Crude Oil [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Crude Oil [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Crude Oil [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Production of Crude Oil [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Processing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	975
Natural Gas Processing [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	975
Natural Gas Processing [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Processing [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Processing [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Processing [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Processing [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Processing [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Gathering Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Gathering Services [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Gathering Services [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Gathering Services [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	29,690
Natural Gas Gathering Services [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Gathering Services [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Gathering Services [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Gathering Services [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(29,690)
Natural Gas Transportation Service [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	74,701
Natural Gas Transportation Service [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Transportation Service [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	56,135
Natural Gas Transportation Service [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Transportation Service [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	35,631
Natural Gas Transportation Service [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Transportation Service [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Transportation Service [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(17,065)
Natural Gas Storage Service [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	10,956
Natural Gas Storage Service [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Storage Service [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	18,929
Natural Gas Storage Service [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Storage Service [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Storage Service [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Storage Service [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Storage Service [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(7,973)
Natural Gas Residential Sales [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	166,867
Natural Gas Residential Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Residential Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Residential Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Residential Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	166,867
Natural Gas Residential Sales [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Residential Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Residential Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Commercial Sales [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	22,047
Natural Gas Commercial Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Commercial Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Commercial Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Commercial Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	22,047
Natural Gas Commercial Sales [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Commercial Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Commercial Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Industrial Sales [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,501
Natural Gas Industrial Sales [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Industrial Sales [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Industrial Sales [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Industrial Sales [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,501
Natural Gas Industrial Sales [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Industrial Sales [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Industrial Sales [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	48,955
Natural Gas Marketing [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Marketing [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Marketing [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Marketing [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Marketing [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	49,287
Natural Gas Marketing [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Natural Gas Marketing [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(332)
Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	129
Other [Member] | Exploration And Production [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	382
Other [Member] | Pipeline And Storage [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	2,005
Other [Member] | Gathering [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Other [Member] | Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	(2,861)
Other [Member] | Energy Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0
Other [Member] | All Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	1,007
Other [Member] | Corporate And Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	$ (404)