Resource Management and Other Commercial Events (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		1 Months Ended	9 Months Ended	1 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Jan. 31, 2014 Nonstrategic Australian mining tenement	Sep. 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	Sep. 30, 2013 Nonstrategic Kentucky coal reserves and surface lands	Sep. 30, 2014 Western U.S. Mining segment customer	Sep. 30, 2014 Positive Outcome of Litigation [Member] Western U.S. Mining segment customer
Other Commercial Events [Line Items]
Contract revenue									$ 33.5
Sales-related expenses								6.4
Proceeds from sale of nonstrategic assets	152.5	133.3	62.6		35.0	35.0
Gain (loss) on sale of nonstrategic asset				0			40.3
Notes receivable from sale of nonstrategic coal reserves						$ 35.0