Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 26,000	$ 31,500
Regulatory Asset, Noncurrent		366,200	362,900
Past expenditures not currently earning a return		48,000	64,000
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current		12,600	12,700
Regulatory Asset, Noncurrent		$ 222,100	223,000
Regulatory asset, remaining amortization period		Various
Excess deferred taxes - TCJA
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 55,900	44,700
Regulatory asset, remaining amortization period		Various
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 27,900	23,900
Regulatory asset, remaining amortization period		Plant lives
Net AROs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[1]	$ 0	0
Regulatory Asset, Noncurrent	[1]	$ 25,700	24,200
Regulatory asset, remaining amortization period		Plant lives
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 800	800
Regulatory Asset, Noncurrent		$ 21,900	22,700
Regulatory asset, remaining amortization period		Term of related debt
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[2]	$ 700	2,700
Regulatory Asset, Noncurrent	[2]	$ 600	700
Regulatory Asset Amortization Period, minimum		1 year
Regulatory Asset Amortization Period, maximum		2 years
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 11,900	15,300
Regulatory Asset, Noncurrent		$ 12,100	$ 23,700
Regulatory asset, remaining amortization period		Various

[1] Includes amounts recorded for future recovery of AROs.

[2] Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.