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Capitalized Exploratory Well Costs (Tables)
 12 Months Ended
Dec. 31, 2015
Capitalized Exploratory Well Costs [Abstract]  
Capitalized Exploratory Well Costs, Roll Forward [Table Text Block]
 
2015
 
2014
 
2013
 
(in millions)
Balance at January 1,
$
12.6

 
$
2.6

 
$
2.1

Additions to capitalized exploratory well costs pending the determination of proved reserves
6.0

 
13.7

 
2.7

Reclassifications to proved properties after the determination of proved reserves
(16.0
)
 
—

 
(2.2
)
Capitalized exploratory well costs charged to expense
—

 
(3.7
)
 
—

Balance at December 31,
$
2.6

 
$
12.6

 
$
2.6