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		$ 344,630	$ 444,058
Regulatory Asset, Noncurrent		2,858,741	2,774,216
Past expenditures not currently earning a return		169,000	323,000
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current	[1]	90,249	95,054
Regulatory Asset, Noncurrent	[1]	$ 1,368,115	1,402,360
Regulatory asset, remaining amortization period		Various
Pension Costs | NSP-Minnesota
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 21,300	23,800
Regulatory Asset		257,500	282,400
Non Qualified Pension Plan
Regulatory Assets [Line Items]
Regulatory Asset, Current		4,000	2,500
Regulatory Asset		12,500	26,100
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current		0	0
Regulatory Asset, Noncurrent		$ 408,994	395,329
Regulatory asset, remaining amortization period		Plant lives
Asset Retirement Obligation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[2]	$ 0	0
Regulatory Asset, Noncurrent	[2]	$ 306,671	189,056
Regulatory asset, remaining amortization period		Plant lives
Contract Valuation Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current	[3]	$ 26,379	17,730
Regulatory Asset, Noncurrent	[3]	$ 128,780	144,273
Regulatory asset, remaining amortization period		Term of related contract
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[4]	$ 31,793	61,866
Regulatory Asset, Noncurrent	[4]	$ 50,047	58,174
Regulatory asset, remaining amortization period, minimum		1 year
Regulatory asset, remaining amortization period, maximum		5 years
Environmental Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 6,702	4,594
Regulatory Asset, Noncurrent		$ 166,883	149,812
Regulatory asset, remaining amortization period		Various
Renewable Resources and Environmental Initiatives
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 33,014	24,891
Regulatory Asset, Noncurrent		$ 23,565	29,902
Regulatory asset, remaining amortization period, minimum		1 year
Regulatory asset, remaining amortization period, maximum		2 years
Depreciation Differences
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 14,221	10,700
Regulatory Asset, Noncurrent		$ 99,835	104,743
Regulatory asset, remaining amortization period, minimum		1 year
Regulatory asset, remaining amortization period, maximum		16 years
Purchased Power Agreements
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 1,587	858
Regulatory Asset, Noncurrent		$ 70,411	69,908
Regulatory asset, remaining amortization period		Term of related contract
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 5,008	5,258
Regulatory Asset, Noncurrent		$ 26,268	31,276
Regulatory asset, remaining amortization period		Term of related debt
Nuclear Refueling Outage Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 67,545	62,499
Regulatory Asset, Noncurrent		$ 28,913	19,745
Regulatory asset, remaining amortization period, minimum		1 year
Regulatory asset, remaining amortization period, maximum		2 years
State Commission Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 988	571
Regulatory Asset, Noncurrent		$ 26,708	26,092
Regulatory asset, remaining amortization period		Plant lives
Gas Pipeline Inspection and Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 6,858	9,981
Regulatory Asset, Noncurrent		$ 13,662	21,869
Regulatory asset, remaining amortization period, minimum		1 year
Regulatory asset, remaining amortization period, maximum		4 years
Deferred Fuel Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 11,783	68,841
Regulatory Asset, Noncurrent		$ 12,762	4,745
Regulatory asset, remaining amortization period, minimum		1 year
Regulatory asset, remaining amortization period, maximum		2 years
Prairie Island EPU
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 2,967	8,743
Regulatory Asset, Noncurrent		$ 65,060	67,379
Regulatory asset, remaining amortization period		19 years
Property Tax
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 21,757	28,024
Regulatory Asset, Noncurrent		$ 14,428	31,429
Regulatory asset, remaining amortization period, minimum		1 year
Regulatory asset, remaining amortization period, maximum		6 years
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 23,779	44,448
Regulatory Asset, Noncurrent		$ 47,639	$ 28,124
Regulatory asset, remaining amortization period		Various

[1] Includes $257.5 million and $282.4 million for the regulatory recognition of the NSP-Minnesota pension expense of which $21.3 million and $23.8 million is included in the current asset at Dec. 31, 2015 and 2014, respectively. Also included are $12.5 million and $26.1 million of regulatory assets related to the nonqualified pension plan of which $4.0 million and $2.5 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, less amounts recovered through nuclear decommissioning accruals and gains from decommissioning investments.

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