Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 38,721	$ 31,541
Regulatory Asset, Noncurrent		346,683	301,814
Past expenditures not currently earning a return		$ 65,000	25,000
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current	[1]	$ 13,986	15,632
Regulatory Asset, Noncurrent	[1]	$ 234,171	223,122
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current		$ 0	0
Regulatory Asset, Noncurrent		$ 44,258	39,368
Net AROs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current	[2]	$ 0	0
Regulatory Asset, Noncurrent	[2]	$ 24,352	23,014
Renewable Resources and Environmental Initiatives
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		One to four years
Regulatory Asset, Current		$ 3,580	3,740
Regulatory Asset, Noncurrent		$ 2,900	2,019
Conservation Programs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		One to three years
Regulatory Asset, Current	[3]	$ 3,754	5,137
Regulatory Asset, Noncurrent	[3]	$ 2,431	3,859
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related debt
Regulatory Asset, Current		$ 127	850
Regulatory Asset, Noncurrent		$ 1,617	1,743
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 17,274	6,182
Regulatory Asset, Noncurrent		$ 36,954	$ 8,689

[1] (a) Includes the non-qualified pension plan.

[2] (b) Includes amounts recorded for future recovery of AROs.

[3] (c) Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.