Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 92,072	$ 120,120
Regulatory Asset, Noncurrent		906,275	903,973
Past expenditures not currently earning a return		$ 54,000	104,000
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current	[1]	$ 29,260	32,195
Regulatory Asset, Noncurrent	[1]	497,973	500,889
Non Qualified Pension Plan
Regulatory Assets [Line Items]
Regulatory Asset, Current		400	400
Regulatory Asset		$ 4,400	4,500
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		$ 144,953	141,214
Contract Valuation Adjustments
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related contract
Regulatory Asset, Current	[2]	$ 9,376	8,901
Regulatory Asset, Noncurrent	[2]	$ 9,526	12,999
Depreciation Differences
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		One to sixteen years
Regulatory Asset, Current		$ 14,221	10,700
Regulatory Asset, Noncurrent		$ 99,835	104,743
Net AROs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Plant lives
Regulatory Asset, Current	[3]	$ 0	0
Regulatory Asset, Noncurrent	[3]	$ 62,948	46,213
Conservation Programs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		One to five years
Regulatory Asset, Current	[4]	$ 8,466	10,198
Regulatory Asset, Noncurrent	[4]	$ 6,947	10,906
Gas Pipeline Inspection Costs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Less than one year
Regulatory Asset, Current		$ 3,611	5,416
Regulatory Asset, Noncurrent		$ 0	3,611
Purchased Power Agreements
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related contract
Regulatory Asset, Current		$ 1,319	858
Regulatory Asset, Noncurrent		$ 29,143	29,596
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Term of related debt
Regulatory Asset, Current		$ 1,421	1,426
Regulatory Asset, Noncurrent		$ 6,957	8,378
Recoverable Purchased Natural Gas and Electric Energy Costs
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Less than one year
Regulatory Asset, Current		$ 408	18,410
Regulatory Asset, Noncurrent		$ 0	0
Property Tax
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		One to six years
Regulatory Asset, Current		$ 21,558	28,024
Regulatory Asset, Noncurrent		$ 14,428	31,429
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period		Various
Regulatory Asset, Current		$ 2,432	3,992
Regulatory Asset, Noncurrent		$ 33,565	$ 13,995

[1] Includes $4.4 million and $4.5 million of regulatory assets related to the nonqualified pension plan, of which $0.4 million is included in the current asset at Dec. 31, 2015 and 2014, respectively.

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

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

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