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Capitalized Exploratory Well Costs
 12 Months Ended
Dec. 31, 2016
Capitalized Exploratory Well Costs [Abstract]  
Suspended Well Costs Disclosure [Text Block]

Net changes in capitalized exploratory well costs are presented in the table below. The balances at December 31, 2016, 2015 and 2014, represent the amount of capitalized exploratory well costs that are pending the determination of proved reserves.
 
Capitalized Exploratory Well Costs
 
2016
 
2015
 
2014
 
(in millions)
Balance at January 1,
$
2.6

 
$
12.6

 
$
2.6

Additions to capitalized exploratory well costs pending the determination of proved reserves
11.7

 
6.0

 
13.7

Reclassifications to proved properties after the determination of proved reserves
—

 
(16.0
)
 
—

Capitalized exploratory well costs charged to expense
(0.1
)
 
—

 
(3.7
)
Balance at December 31,
$
14.2

 
$
2.6

 
$
12.6