Restructuring and Asset Impairment (Details) (USD $)	3 Months Ended	12 Months Ended						3 Months Ended	6 Months Ended	3 Months Ended
	Sep. 30, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 North River mine	Dec. 31, 2012 North River mine	Dec. 31, 2013 Willow Creek mine	Dec. 31, 2012 Willow Creek mine	Dec. 31, 2013 North River Mine Closure	Jun. 30, 2013 Willow Creek Mine Closure	Dec. 31, 2012 Aberpergwm underground coal mine Closure
Asset impairment and restructuring
Gain on release of a below market contract liability								$ 17,000,000
Asset impairment charges			1,107,512,000					9,300,000		3,100,000
Curtailment costs									10,700,000
Restructuring and impairment charge		2,883,000	1,113,479,000	(7,763,000)	114,281,000	10,646,000	999,198,000			9,100,000
Severance and other obligations										6,000,000
Write-off capitalized exploratory costs associated with natural gas exploration project, net of taxes	40,000,000
Write-off capitalized exploratory costs associated with natural gas exploration project, net of taxes	$ 25,000,000