Asset Retirement Obligations (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligations [Roll Forward]
Asset retirement obligation, beginning balance		$ 24,360	$ 19,744
Liabilities incurred		1,303	2,380	[1]
Accretion expense		1,269	1,131
Liabilities settled		(110)	(124)
Revisions in estimated cash flows		64	1,229	[1]
Asset retirement obligation, ending balance		$ 26,886	24,360
Columbia Units [Member]
Asset Retirement Obligations [Roll Forward]
Liabilities incurred	[1]		2,300
Revisions in estimated cash flows	[1]		$ 1,300

[1] In the second quarter of 2015 , M GE recorded an obligation of $2.3 million for the fair value of its legal liability for AROs associated with the effect of the final Coal Combustion Residual Rule at Columbia. An additional $1.3 million was recorded in the fourth quarter, associated with this ARO, based on revised estimates.