Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 2,608,562		$ 2,446,631
Liabilities Incurred	18,135		35,087
Liabilities Settled	(6,300)		(273)
Accretion	127,703		120,119
Cash flow revisions	34,129	[1]	6,998	[2]
Ending balance	2,782,229		2,608,562
Legally restricted assets, for purposes of funding future nuclear decommissioning			1,700,000
Electric Plant Nuclear Production Decommissioning
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	2,141,024		2,037,947
Liabilities Incurred	0		0
Liabilities Settled	0		0
Accretion	108,298		103,077
Cash flow revisions	0	[1]	0	[2]
Ending balance	2,249,322		2,141,024
Electric Plant Steam and Other Production Ash Containment
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	131,587		127,600
Liabilities Incurred	0		0
Liabilities Settled	(6,271)		0
Accretion	4,913		4,746
Cash flow revisions	(13,843)	[1]	(759)	[2]
Ending balance	116,386		131,587
Electric Plant Steam and Other Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	84,491		69,698
Liabilities Incurred	0		3,875
Liabilities Settled	0		0
Accretion	4,054		3,670
Cash flow revisions	(103)	[1]	7,248	[2]
Ending balance	88,442		84,491
Electric Plant Wind Production
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	71,646		38,260
Liabilities Incurred	17,305	[3]	31,085	[4]
Liabilities Settled	0		0
Accretion	3,166		1,778
Cash flow revisions	61	[1]	523	[2]
Ending balance	92,178		71,646
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	13,187		12,593
Liabilities Incurred	0		0
Liabilities Settled	0		0
Accretion	485		463
Cash flow revisions	6,451	[1]	131	[2]
Ending balance	20,123		13,187
Electric Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	4,543		4,605
Liabilities Incurred	645		127
Liabilities Settled	(29)		(273)
Accretion	176		178
Cash flow revisions	(451)	[1]	(94)	[2]
Ending balance	4,884		4,543
Natural Gas Plant Gas Transmission and Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	155,933		149,964
Liabilities Incurred	0		0
Liabilities Settled	0		0
Accretion	6,368		5,969
Cash flow revisions	42,483	[1]	0	[2]
Ending balance	204,784		155,933
Natural Gas Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	3,966		3,925
Liabilities Incurred	185		0
Liabilities Settled	0		0
Accretion	158		155
Cash flow revisions	0	[1]	(114)	[2]
Ending balance	4,309		3,966
Common and Other Property Common General Plant Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	551		505
Liabilities Incurred	0		0
Liabilities Settled	0		0
Accretion	28		27
Cash flow revisions	0	[1]	19	[2]
Ending balance	579		551
Common and Other Property Common Miscellaneous
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,634		1,534
Liabilities Incurred	0		0
Liabilities Settled	0		0
Accretion	57		56
Cash flow revisions	(469)	[1]	44	[2]
Ending balance	$ 1,222		$ 1,634

[1] (b) In 2016, AROs were revised for changes in estimated cash flows and the timing of those cash flows. Changes in the gas transmission and distribution AROs were mainly related to increased miles of gas mains.

[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) The liability recognized relates to the NSP-Minnesota Courtenay Wind Farm which was placed in service during 2016.

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