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		$ 103,783	$ 92,072
Regulatory Asset, Noncurrent		958,429	906,275
Past expenditures not currently earning a return		$ 28,000	54,000
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current	[1]	$ 27,270	29,260
Regulatory Asset, Noncurrent	[1]	568,258	497,973
Non Qualified Pension Plan
Regulatory Assets [Line Items]
Regulatory Asset, Current		400	400
Regulatory Asset		$ 4,200	4,400
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		$ 151,022	144,953
Depreciation Differences
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Pending rate case
Regulatory Asset, Current		$ 15,363	14,221
Regulatory Asset, Noncurrent		$ 90,426	99,835
Net AROs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current	[2]	$ 0	0
Regulatory Asset, Noncurrent	[2]	$ 78,050	62,948
Purchased Power Agreements
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related contract
Regulatory Asset, Current		$ 1,035	1,319
Regulatory Asset, Noncurrent		$ 29,029	29,143
Conservation Programs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		One to two years
Regulatory Asset, Current	[3]	$ 9,262	8,466
Regulatory Asset, Noncurrent	[3]	$ 6,986	6,947
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related debt
Regulatory Asset, Current		$ 1,203	1,421
Regulatory Asset, Noncurrent		$ 6,120	6,957
Contract Valuation Adjustments
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related contract
Regulatory Asset, Current	[4]	$ 3,444	9,376
Regulatory Asset, Noncurrent	[4]	$ 6,082	9,526
Gas Pipeline Inspection Costs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Two years
Regulatory Asset, Current		$ 0	3,611
Regulatory Asset, Noncurrent		$ 4,405	0
Property Tax
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 9,393	21,558
Regulatory Asset, Noncurrent		$ 1,653	14,428
CACJA Recovery Rider
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Less than one year
Regulatory Asset, Current		$ 24,260	0
Regulatory Asset, Noncurrent		$ 0	20,020
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 12,553	2,840
Regulatory Asset, Noncurrent		$ 16,398	$ 13,545

[1] (a) Includes $4.2 million and $4.4 million of regulatory assets related to the nonqualified pension plan, of which $0.4 million is included in the current asset at Dec. 31, 2016 and 2015, respectively.

[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.

[4] (d) Includes the fair value of certain long-term PPAs used to meet energy capacity requirements and valuation adjustments on natural gas commodity purchases.