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	$ 240,508	[1]	$ 225,296
Liabilities recognized	214		0
Liabilities settled	0		0
Accretion	10,257		9,575
Cash Flow Revisions	38,584	[2]	5,637	[3]
Ending balance	289,563	[4]	240,508	[1]
Electric Plant Steam and Other Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	38,676	[1]	36,856
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	1,877		1,820
Cash Flow Revisions	(103)	[2]	0	[3]
Ending balance	40,450	[4]	38,676	[1]
Electric Plant Steam and Other Production Ash Containment
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	70,767	[1]	61,885
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	3,078		2,769
Cash Flow Revisions	(1,245)	[2]	6,113	[3]
Ending balance	72,600	[4]	70,767	[1]
Electric Plant Wind Production
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,992	[1]	2,095
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	19		18
Cash Flow Revisions	61	[2]	(121)	[3]
Ending balance	2,072	[4]	1,992	[1]
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,130	[1]	1,182
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	45		47
Cash Flow Revisions	6,494	[2]	(99)	[3]
Ending balance	7,669	[4]	1,130	[1]
Electric Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,054	[1]	1,150
Liabilities recognized	214		0
Liabilities settled	0		0
Accretion	46		46
Cash Flow Revisions	206	[2]	(142)	[3]
Ending balance	1,520	[4]	1,054	[1]
Natural Gas Plant Gas Transmission and Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	122,168	[1]	117,474
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	5,009		4,694
Cash Flow Revisions	33,542	[2]	0	[3]
Ending balance	160,719	[4]	122,168	[1]
Natural Gas Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	3,925	[1]	3,886
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	155		153
Cash Flow Revisions	0	[2]	(114)	[3]
Ending balance	4,080	[4]	3,925	[1]
Common and Other Property Common Miscellaneous
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	796	[1]	768
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	28		28
Cash Flow Revisions	(371)	[2]	0	[3]
Ending balance	$ 453	[4]	$ 796	[1]

[1] (b) There were no ARO liabilities recognized or settled during the year ended Dec. 31, 2015.

[2] (a) 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.

[3] (a) In 2015, AROs were revised for changes in estimated cash flows and the timing of those cash flows. Changes in the ash containment ARO were mainly related to the final coal ash rule mentioned above.

[4] (b) There were no ARO liabilities settled during the year ended Dec. 31, 2016.