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Property, Plant, Equipment and Mine Development (Tables)
 12 Months Ended
Dec. 31, 2018
Property, Plant, Equipment and Mine Development [Abstract]  
Property, Plant and Equipment [Table Text Block]
Property, plant, equipment and mine development, net, as of December 31, 2018 and December 31, 2017 consisted of the following:
 
December 31, 2018
 
December 31, 2017
 
(Dollars in millions)
Land and coal interests
$
4,148.8

 
$
3,890.5

Buildings and improvements
559.3

 
470.6

Machinery and equipment
1,456.3

 
1,149.3

Less: Accumulated depreciation, depletion and amortization
(957.4
)
 
(398.5
)
Property, plant, equipment and mine development, net
$
5,207.0

 
$
5,111.9