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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance on January 1, $ 47,386 $ 51,851
Liabilities Incurred 828 413
Liabilities Settled (4,693) (1,017)
Accretion Expense 2,560 2,186
Revision of Prior Estimate [1] (1,346) (6,047) [2]
Balance on December 31, 44,735 47,386
Electric Utilities [Member]    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance on January 1, 7,012 6,922
Liabilities Incurred 0 0
Liabilities Settled (2,733) (85)
Accretion Expense 183 175
Revision of Prior Estimate 0 0
Balance on December 31, 4,462 7,012
Gas Utilities [Member]    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance on January 1, 291 274
Liabilities Incurred 0 0
Liabilities Settled (168) 0
Accretion Expense 13 17
Revision of Prior Estimate 0 0
Balance on December 31, 136 291
Coal Mining [Member]    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance on January 1, 19,138 20,627
Liabilities Incurred 0 345
Liabilities Settled 0 0
Accretion Expense 993 951
Revision of Prior Estimate [1] (1,498) (2,785)
Balance on December 31, 18,633 19,138
Oil and Gas [Member]    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance on January 1, 20,945 24,028
Liabilities Incurred 828 68
Liabilities Settled (1,792) (932)
Accretion Expense 1,371 1,043
Revision of Prior Estimate 152 (3,262) [2]
Balance on December 31, $ 21,504 $ 20,945
[1] The Coal Mining Revision to Prior Estimates reflects the change in backfill yards and disturbed acreage used in calculating the estimated liability as well as changes in inflation rate assumptions.
[2] The Oil and Gas Revision to Prior Estimates was due to a change in useful well lives used in calculating the estimated liability.