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Wexpro and Wexpro II Agreements (Tables)
 12 Months Ended
Dec. 31, 2014
Wexpro Agreement [Abstract]  
Net investment base, yearly average rate of return and oil and NGL income sharing
Wexpro's net investment base, yearly average rate of return, and oil and NGL income shared with Questar Gas are shown in the table below:
 
Year Ended December 31,
 
2014
 
2013
 
2012
Wexpro net investment base at December 31, (in millions)
$
649.0

 
$
589.7

 
$
531.1

Average annual rate of return (after tax)
17.9
%
 
19.7
%
 
19.9
%
Oil and NGL income sharing (in millions)
$
—

 
$
0.6

 
$
2.5