Regulatory Matters (Narrative) (Details) - USD ($) $ in Thousands, number in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Regulated Asset Liability [Line Items]
Company Share of Benefit (Expense) Under Washington Energy Recovery Mechanism	$ 6,100	$ 4,600
Deferred natural gas costs	$ 40,700	37,500
WASHINGTON
Regulated Asset Liability [Line Items]
Decoupling Maximum Rate Increase Request	0.00%
Power Deferrals [Member] | WASHINGTON
Regulated Asset Liability [Line Items]
Regulatory Liabilities	$ (34,400)	(23,700)
Power Deferrals [Member] | IDAHO
Regulated Asset Liability [Line Items]
Regulatory Liabilities	(7,600)	(6,100)
Revenue Subject to Refund [Member] | WASHINGTON
Regulated Asset Liability [Line Items]
Regulatory Liabilities	(693)	(3,420)
Revenue Subject to Refund [Member] | IDAHO
Regulated Asset Liability [Line Items]
Regulatory Liabilities	$ (774)	$ (2,350)