Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 22, 2017	Sep. 30, 2017
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 1,510,997		$ 555,657
Regulatory Assets [Line Items]
Regulatory Assets	143,811		176,798
Pension and postretirement expense deferred	6,500		9,400
Deferred tax liability, regulated operations	744,900	$ 746,500
Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities			0
Regulatory Cost of Service Reserve
Regulatory Liabilities [Line Items]
Regulatory Liabilities	22,508		0
Regulatory cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	522,175		521,330
Deferred gas costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	94,705		15,559
Asset retirement obligation
Regulatory Liabilities [Line Items]
Regulatory Liabilities	12,887		12,827
APT annual adjustment mechanism
Regulatory Liabilities [Line Items]
Regulatory Liabilities	35,228		0
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	69,113		0
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities	9,486		5,941
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory Assets	6,496		26,826
Infrastructure mechanisms
Regulatory Assets [Line Items]
Regulatory Assets	96,739		46,437
Deferred gas costs
Regulatory Assets [Line Items]
Regulatory Assets	1,927		65,714
Recoverable loss on reacquired debt
Regulatory Assets [Line Items]
Regulatory Assets	8,702		11,208
Deferred pipeline record collection costs
Regulatory Assets [Line Items]
Regulatory Assets	20,467		11,692
APT annual adjustment mechanism
Regulatory Assets [Line Items]
Regulatory Assets	0		2,160
Rate case costs
Regulatory Assets [Line Items]
Regulatory Assets	2,741		2,629
Other
Regulatory Assets [Line Items]
Regulatory Assets	6,739		$ 10,132
Other Current Liabilities | Regulatory excess deferred taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 5,200