Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from Tax Reform (Note 1)		$ 471,096	$ 0	$ 0
Purchase Price Allocation
Purchase price allocation, gross		$ 1,500,000
Purchase price allocation, property, plant and equipment, estimated useful lIfe		40 years
Purchase price allocation, depreciation		$ 35,000
Purchase price allocation, remaining allocation		600,000
Schedule of Equity Method Investments [Line Items]
Proceeds from Equity Method Investment, Distribution		8,000	8,600	7,600
Proceeds from Equity Method Investment, Distribution, Return of Capital		3,926	2,767	2,015
Capitalized Interest Costs, Including Allowance for Funds Used During Construction [Abstract]
Allowance for funds used during construction, borrowed		22,300	12,500	14,600
Allowance for funds used during construction, equity		69,653	56,468	$ 48,435
Inventory Disclosure [Abstract]
Gas in storage, LIFO		0	0
Excess of replacement cost over LIFO cost		0	$ 0
Operating Expense [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from Tax Reform (Note 1)		471,100
Nonoperating Income (Expense) [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from Tax Reform (Note 1)		10,300
Other Nonoperating Income (Expense) [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge resulting from Tax Reform (Note 1)		$ 32,700
Minimum
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		21.00%
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Description of Principal Activities		.20
Maximum
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		35.00%
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Description of Principal Activities		.50
Pine Needle LNG Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		35.00%	35.00%
Cardinal Pipeline Company LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		45.00%	45.00%
Financial Repositioning [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Parent, limited partner ownership percentage		74.00%
Parent, general partner ownership percentage	2.00%
Gathering facilities | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation rates		1.35%	1.35%	1.35%
Gathering facilities | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation rates		2.50%	2.50%	2.50%
Storage facilities | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation rates		2.10%	2.10%	2.10%
Storage facilities | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation rates		2.25%	2.25%	2.25%
Onshore transmission facilities | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation rates		2.61%	2.61%	2.61%
Onshore transmission facilities | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation rates		5.00%	5.00%	5.00%
Offshore transmission facilities | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation rates		1.20%	1.20%	1.20%
Offshore transmission facilities | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation rates		1.20%	1.20%	1.20%