Utility Plant - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent	$ 3,000	$ 2,700
Subsidiaries [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent	12,400	4,800
Subsidiaries [Member] | Colstrip Units 1 and 2 [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent	4,200	11,000
Subsidiaries [Member] | Colstrip Units 3 and 4 [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent	500	1,800
Puget LNG [Member] | Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent	$ 4,300	$ 1,700