Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 2,446,631,000	[1]	$ 1,815,390,000
Liabilities recognized	35,087,000		25,252,000
Liabilities Settled	(273,000)		0
Accretion	120,119,000		95,045,000
Cash flow revisions	6,998,000	[2]	510,944,000	[3]
Ending balance	2,608,562,000		2,446,631,000	[1]
Legally restricted assets, for purposes of funding future nuclear decommissioning			1,700,000,000
Electric Plant Nuclear Production Decommissioning
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	2,037,947,000	[1]	1,628,298,000
Liabilities recognized	0		0
Liabilities Settled	0		0
Accretion	103,077,000		86,284,000
Cash flow revisions	0	[2]	323,365,000	[3]
Ending balance	2,141,024,000		2,037,947,000	[1]
Electric Plant Steam and Other Production Ash Containment
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	127,600,000	[1]	79,353,000
Liabilities recognized	0		0
Liabilities Settled	0		0
Accretion	4,746,000		3,354,000
Cash flow revisions	(759,000)	[2]	44,893,000	[3]
Ending balance	131,587,000		127,600,000	[1]
Electric Plant Steam and Other Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	69,698,000	[1]	50,827,000
Liabilities recognized	3,875,000		747,000
Liabilities Settled	0		0
Accretion	3,670,000		2,972,000
Cash flow revisions	7,248,000	[2]	15,152,000	[3]
Ending balance	84,491,000		69,698,000	[1]
Electric Plant Wind Production
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	38,260,000	[1]	37,464,000
Liabilities recognized	31,085,000	[4]	0
Liabilities Settled	0		0
Accretion	1,778,000		1,676,000
Cash flow revisions	523,000	[2]	(880,000)	[3]
Ending balance	71,646,000		38,260,000	[1]
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	12,593,000	[1]	12,186,000
Liabilities recognized	0		0
Liabilities Settled	0		0
Accretion	463,000		444,000
Cash flow revisions	131,000	[2]	(37,000)	[3]
Ending balance	13,187,000		12,593,000	[1]
Electric Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	4,605,000	[1]	3,551,000
Liabilities recognized	127,000		705,000
Liabilities Settled	(273,000)		0
Accretion	178,000		137,000
Cash flow revisions	(94,000)	[2]	212,000	[3]
Ending balance	4,543,000		4,605,000	[1]
Natural Gas Plant Gas Transmission and Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	149,964,000	[1]	1,198,000
Liabilities recognized	0		20,935,000
Liabilities Settled	0		0
Accretion	5,969,000		76,000
Cash flow revisions	0	[2]	127,755,000	[3]
Ending balance	155,933,000		149,964,000	[1]
Natural Gas Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	3,925,000	[1]	575,000
Liabilities recognized	0		2,865,000
Liabilities Settled	0		0
Accretion	155,000		24,000
Cash flow revisions	(114,000)	[2]	461,000	[3]
Ending balance	3,966,000		3,925,000	[1]
Common and Other Property Common General Plant Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	505,000	[1]	480,000
Liabilities recognized	0		0
Liabilities Settled	0		0
Accretion	27,000		25,000
Cash flow revisions	19,000	[2]	0	[3]
Ending balance	551,000		505,000	[1]
Common and Other Property Common Miscellaneous
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,534,000	[1]	1,458,000
Liabilities recognized	0		0
Liabilities Settled	0		0
Accretion	56,000		53,000
Cash flow revisions	44,000	[2]	23,000	[3]
Ending balance	$ 1,634,000		$ 1,534,000	[1]

[1] (b) There were no ARO liabilities settled during the year ended Dec. 31, 2014.

[2] (b) In 2015, AROs were revised for changes in estimated cash flows and the timing of those cash flows. Changes in the asbestos AROs were mainly related to updated cost estimates.

[3] (a) In 2014, revisions were made to various AROs due to revised estimated cash flows and the timing of those cash flows. Changes to estimated nuclear production decommissioning primarily relate to the triennial filing made to the MPUC in December 2014. See additional information in Note 14. Changes in estimated excavation costs and the timing of future retirement activities resulted in revisions to AROs related to gas transmission and distribution.

[4] (a) The liability recognized relates to the NSP-Minnesota Pleasant Valley and Border Wind Farms which were placed in service during 2015.