Summary of Significant Accounting Policies (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Entity number of employees		0				0
Regulatory charge resulting from Tax Reform (Note 1)		$ 206,547,000	$ 0	$ 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		4,857,000	3,510,000	2,550,000
Regulatory assets			34,900,000		22,747,000
Capitalized Interest Costs, Including Allowance for Funds Used During Construction [Abstract]
Allowance for funds used during construction, debt		300,000	400,000	500,000
Allowance for funds used during construction, equity		1,104,000	953,000	1,099,000
Interest Payments [Abstract]
Cash payments for interest, net of interest capitalized		31,900,000	38,500,000	45,300,000
Levelized depreciation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory debits		4,900,000	3,500,000	$ 2,600,000
Regulatory assets			21,849,000		16,149,000
Asset retirement obligation [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets			78,500,000		82,200,000
Negative salvage [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Regulatory assets			$ (78,500,000)		$ (82,200,000)
Operating Expense [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from Tax Reform (Note 1)		206,547,000
Other Nonoperating Income (Expense) [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from Tax Reform (Note 1)		$ 5,700,000
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%
Financial Repositioning [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Parent, general partner ownership percentage	2.00%
Parent, limited partner ownership percentage	74.00%
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%