Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge (credit) resulting from Tax Reform (Note 1)	$ (20,867)	$ 471,096	$ 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	3,250	8,036	8,631
Capitalized Interest Costs, Including Allowance for Funds Used During Construction [Abstract]
Allowance for funds used during construction, borrowed	29,200	22,300	12,500
Allowance for funds used during construction, equity	87,111	69,653	$ 56,468
Inventory Disclosure [Abstract]
Gas in storage, LIFO	0	0
Operating Expense [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge (credit) resulting from Tax Reform (Note 1)	(20,867)	471,096
Nonoperating Income (Expense) [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge (credit) resulting from Tax Reform (Note 1)	2,000	10,300
Other Nonoperating Income (Expense) [Member]
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Regulatory charge (credit) resulting from Tax Reform (Note 1)	$ 900	$ 32,700
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%
Minimum
Schedule of Equity Method Investments [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		21.00%
Equity Method Investment, Description of Principal Activities	.20
Maximum
Schedule of Equity Method Investments [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		35.00%
Equity Method Investment, Description of Principal Activities	.50
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%
Accounting Standards Update 2016-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by (Used in) Operating Activities		$ 3,900	$ 2,800
Accounting Standards Update 2016-02 [Member] | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease Liability Percentage Of Total Liabilities	3.00%
Operating Lease Right Of Use Asset Percentage Of Total Assets	3.00%