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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2015
Asset Retirement Obligation [Abstract]  
Changes in AROs
Changes in Questar's AROs from the Condensed Consolidated Balance Sheets were as follows:
 
9 Months Ended
 
September 30,
 
2015
 
2014
 
(in millions)
AROs at beginning of year
$
69.3

 
$
67.7

Accretion
2.5

 
2.5

Liabilities incurred
0.6

 
0.7

Revisions in estimated cash flows
5.7

 
0.4

Liabilities settled
(7.1
)
 
(2.1
)
AROs at end of period
$
71.0

 
$
69.2