Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 225,296	[1]	$ 60,398
Liabilities recognized	0		21,864
Liabilities settled	0		0
Accretion	9,575		3,701
Cash Flow Revisions	5,637	[2]	139,333	[3]
Ending balance	240,508	[4]	225,296	[1]
Electric Plant Steam and Other Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	36,856	[1]	23,914
Liabilities recognized	0		747
Liabilities settled	0		0
Accretion	1,820		1,597
Cash Flow Revisions	0	[2]	10,598	[3]
Ending balance	38,676	[4]	36,856	[1]
Electric Plant Steam and Other Production Ash Containment
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	61,885	[1]	29,234
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	2,769		1,897
Cash Flow Revisions	6,113	[2]	30,754	[3]
Ending balance	70,767	[4]	61,885	[1]
Electric Plant Wind Production
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	2,095	[1]	2,953
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	18		22
Cash Flow Revisions	(121)	[2]	(880)	[3]
Ending balance	1,992	[4]	2,095	[1]
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,182	[1]	1,176
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	47		43
Cash Flow Revisions	(99)	[2]	(37)	[3]
Ending balance	1,130	[4]	1,182	[1]
Electric Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,150	[1]	1,017
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	46		41
Cash Flow Revisions	(142)	[2]	92	[3]
Ending balance	1,054	[4]	1,150	[1]
Natural Gas Plant Gas Transmission and Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	117,474	[1]	788
Liabilities recognized	0		18,252
Liabilities settled	0		0
Accretion	4,694		50
Cash Flow Revisions	0	[2]	98,384	[3]
Ending balance	122,168	[4]	117,474	[1]
Natural Gas Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	3,886	[1]	575
Liabilities recognized	0		2,865
Liabilities settled	0		0
Accretion	153		24
Cash Flow Revisions	(114)	[2]	422	[3]
Ending balance	3,925	[4]	3,886	[1]
Common and Other Property Common Miscellaneous
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	768	[1]	741
Liabilities recognized	0		0
Liabilities settled	0		0
Accretion	28		27
Cash Flow Revisions	0	[2]	0	[3]
Ending balance	$ 796	[4]	$ 768	[1]

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

[2] 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.

[3] In 2014, revisions were made to various AROs due to revised estimated cash flows and the timing of those cash flows. Changes in estimated excavation costs and the timing of future retirement activities resulted in revisions to AROs related to gas transmission and distribution.

[4] There were no ARO liabilities recognized or settled during the year ended Dec. 31, 2015.