Summary of Consolidation Policy (Details) $ in Millions	Sep. 30, 2017 USD ($) mi²
Subsidiaries [Member]
Summary of Consolidation Policy
Area of Service Territory (in sqmi) | mi²	6,000
Subsidiaries [Member] | Tacoma LNG [Member]
Summary of Consolidation Policy
Jointly Owned Non-Utility Plant Share	43.00%
Construction in Progress, Gross	$ 76.3
Puget LNG [Member]
Summary of Consolidation Policy
Construction in Progress and O&M Expenses	$ 86.5
Jointly Owned Non-Utility Plant Share	57.00%
Construction in Progress, Gross	$ 86.4