Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) employee	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Entity number of employees | employee	0
Regulatory charge resulting from tax rate change	$ 35,680,000	$ 206,547,000	$ 0
Revenue Recognition [Abstract]
Reserve for rate refund, Current	0
Reserve for rate refund, Noncurrent	0
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory debits	2,334,000	4,857,000	3,510,000
Regulatory assets	23,992,000	22,747,000
Capitalized Interest Costs, Including Allowance for Funds Used During Construction [Abstract]
Allowance for funds used during construction, debt	500,000	300,000	400,000
Allowance for funds used during construction, equity	1,703,000	1,104,000	953,000
Interest Payments [Abstract]
Cash payments for interest, net of interest capitalized	25,400,000	31,900,000	38,500,000
State Tax [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from tax rate change	12,100,000
Levelized depreciation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory debits	2,300,000	4,900,000	$ 3,500,000
Regulatory assets	18,109,000	16,149,000
Asset retirement obligation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets	86,400,000	82,200,000
Negative salvage [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets	$ 86,400,000	$ 82,200,000
Minimum [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Effective Income Tax Rate Reconciliation, Deduction, Percent		21.00%
Maximum [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Effective Income Tax Rate Reconciliation, Deduction, Percent		35.00%
Maximum [Member] | Accounting Standards Update 2016-02 [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Operating Lease Liability Percentage Of Total Liabilities	3.00%
Operating Lease Right Of Use Asset Percentage Of Total Assets	3.00%
Storage Facilities [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciaton rates	1.60%	1.60%	1.60%
Storage Facilities [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciaton rates	2.76%	2.76%	2.76%
Transmission Facilities [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciaton rates	2.80%	2.80%	2.80%
Transmission Facilities [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciaton rates	6.97%	6.97%	6.97%
Other Nonoperating Income (Expense) [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from tax rate change		$ 5,700,000
Operating Expense [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from tax rate change	$ 206,547,000