Resource Management and Other Commercial Events (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		1 Months Ended	6 Months Ended	1 Months Ended		3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jan. 31, 2014 Nonstrategic Australian mining tenement	Jun. 30, 2014 Nonstrategic Australian mining tenement	Dec. 31, 2013 Nonstrategic Kentucky coal reserves and surface lands	Jun. 30, 2013 Nonstrategic Kentucky coal reserves and surface lands	Jun. 30, 2013 Nonstrategic Kentucky coal reserves and surface lands	Jun. 30, 2013 Nonstrategic Kentucky coal reserves and surface lands	Jun. 30, 2014 Western U.S. Mining segment customer	Jun. 30, 2014 Western U.S. Mining segment customer	Jun. 30, 2014 Positive Outcome of Litigation [Member] Western U.S. Mining segment customer	Jun. 30, 2014 Positive Outcome of Litigation [Member] Western U.S. Mining segment customer
Other Commercial Events [Line Items]
Contract revenue											$ 43.2	$ 33.5
Sales and production taxes and royalties									8.2	6.4
Proceeds from sale of nonstrategic assets	139.2	123.9	62.6		35.0	35.0
Notes receivable from sale of nonstrategic coal reserves						35.0	35.0	35.0
Gain (loss) on sale of nonstrategic asset				$ 0			$ 40.3	$ 40.3