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Capitalized Exploratory Well Costs (Tables)
 12 Months Ended
Dec. 31, 2014
Capitalized Exploratory Well Costs [Abstract]  
Capitalized Exploratory Well Costs, Roll Forward [Table Text Block]
 
2014
 
2013
 
2012
 
(in millions)
Balance at January 1,
$
2.6

 
$
2.1

 
$
5.0

Additions to capitalized exploratory well costs pending the determination of proved reserves
13.7

 
2.7

 
12.7

Reclassifications to proved properties after the determination of proved reserves
—

 
(2.2
)
 
(15.6
)
Capitalized exploratory well costs charged to expense
(3.7
)
 
—

 
—

Balance at December 31,
$
12.6

 
$
2.6

 
$
2.1