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Accounting for Asset Retirement Obligations Accounting for Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2012
Accounting for Asset Retirement Obligations [Abstract]  
Changes In Estimated Probabilities Assumptions
The assumptions used to calculate the original Palo Verde ARO liability and the revised Palo Verde ARO liability are as follows: 
        
 
Escalation
Rate
 
Credit-Risk
Adjusted
Discount Rate
Original ARO liability
3.60
%
 
9.50
%
Incremental ARO liability
3.60
%
 
6.20
%
Schedule of Asset Retirement Obligations
A roll forward of the Company's ARO liability is presented below and revisions to estimates include both the increase to estimated cash flows and the change in estimated probabilities for life extension due to the approval of a Palo Verde license extension in 2011 and the change in the estimated probability of extending the Four Corners' original lease term in 2012.

        
 
2012
 
2011
 
2010
ARO liability at beginning of year
$
56,140

 
$
92,911

 
$
85,358

Liabilities incurred
—

 
—

 
—

Liabilities settled
(450
)
 
(793
)
 
(85
)
Revisions to estimate
1,929

 
(41,670
)
 
(377
)
Accretion expense
5,165

 
5,692

 
8,015

ARO liability at end of year
$
62,784

 
$
56,140

 
$
92,911