Utility Plant - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent	$ 3.9	$ 3.8
Subsidiaries [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent	6.9	13.1
Subsidiaries [Member] | Colstrip Units 1 and 2 [Member]
Jointly Owned Utility Plant Interests
Regulatory Assets, Unrecovered plant balance			$ 126.5
Decommissioning Liability, Noncurrent	0.0	1.5
Subsidiaries [Member] | Colstrip Units 3 and 4 [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent		3.1
Puget LNG [Member] | Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Decommissioning Liability, Noncurrent	$ 3.8	$ 3.7