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Accounting for Asset Retirement Obligations Accounting for Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2011
Accounting for Asset Retirement Obligations [Abstract]  
Changes In Estimated Probabilities Assumptions
The assumptions used to calculate the original ARO liability and the revised 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 due to Palo Verde license extension.

        
 
2011
 
2010
 
2009
ARO liability at beginning of year
$
92,911

 
$
85,358

 
$
78,037

Liabilities incurred
—

 
—

 
—

Liabilities settled
(793
)
 
(85
)
 
—

Revisions to estimate
(41,670
)
 
(377
)
 
—

Accretion expense
5,692

 
8,015

 
7,321

ARO liability at end of year
$
56,140

 
$
92,911

 
$
85,358