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Area of Service Territory

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Construction in Progress and O&M Expenses

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Monetization of production tax credits

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Regulated Utility, Allowed Rate of Return on Net Regulatory Assets and Liabilities

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Self Insurance [Abstract]

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A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service.

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The excise taxes collected by a public utility and reported in revenues if the amount of excise taxes is equal to 1 percent or more of such total.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Percentage of public utility's approved rate increase (decrease) by regulatory agency.

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Summary of Significant Accounting Policies (Details) $ in Thousands				6 Months Ended	12 Months Ended
	Nov. 07, 2018	Dec. 19, 2017	Dec. 18, 2017	Dec. 31, 2013	Dec. 31, 2018 USD ($) mi² unit	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)	Jan. 01, 2017 USD ($)	Feb. 06, 2009 USD ($)
Accounting Policies
Monetized production tax credits					$ 84,000	$ 2,100
Goodwill					$ 1,656,513	1,656,513				$ 1,700,000
Number of reportable segments \| unit					1
Cash and Cash Equivalents
Cash and Cash Equivalents, at Carrying Value					$ 37,521	26,616
Revenue Recognition
Excise taxes collected					239,300	257,100	$ 235,300
Allowance for Doubtful Accounts
Allowance for doubtful accounts					$ 8,408	$ 8,901
Electric Transmission
Accounting Policies
Annual depreciation provision					3.30%	2.80%	2.80%
Gas Transmission Equipment
Accounting Policies
Annual depreciation provision					2.80%	3.40%	3.40%
Common Plant
Accounting Policies
Annual depreciation provision					7.10%	8.30%	9.70%
Subsidiaries [Member]
Accounting Policies
Regulated Utility, Allowed Rate of Return on Net Regulatory Assets and Liabilities	7.49%	7.60%		7.77%
Area of service territory (sqmi) \| mi²					6,000
Cash and Cash Equivalents
Cash and Cash Equivalents, at Carrying Value					$ 35,452	$ 25,864
Allowance for Doubtful Accounts
Allowance for doubtful accounts					$ 8,408	$ 8,901
Self Insurance [Abstract]
Public Utilities, Rate Case, Deferred Storm Costs Threshold								$ 10,000	$ 8,000
Puget LNG [Member]
Statements of Cash Flows
Jointly Owned Non-Utility Plant Share					57.00%
Construction in Progress, Gross					$ 165,600
Construction in Progress and O&M Expenses					$ 130,800
Tacoma LNG [Member]
Statements of Cash Flows
Jointly Owned Non-Utility Plant Share					43.00%
Construction in Progress, Gross					$ 2,000
Decoupling Mechanism [Member] \| Electricity, US Regulated [Member] \| Subsidiaries [Member] \| Maximum
Revenue Recognition
Public Utilities, Approved Rate Increase (Decrease), Percentage		3.00%
Decoupling Mechanism [Member] \| Natural Gas, US Regulated [Member] \| Subsidiaries [Member] \| Maximum
Revenue Recognition
Public Utilities, Approved Rate Increase (Decrease), Percentage		5.00%	3.00%