Quarterly Financial Information (Details) (USD $)	3 Months Ended								12 Months Ended							3 Months Ended			1 Months Ended	3 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Parent/Issuer [Member]	Dec. 31, 2010 Parent/Issuer [Member]	Dec. 31, 2009 Parent/Issuer [Member]	Oct. 01, 2009 Jacobs Ranch Mining Operations [Member]	Dec. 31, 2011 ICG [Member]	Sep. 30, 2011 ICG [Member]	Jun. 30, 2011 ICG [Member]	Jun. 30, 2010 Knight Hawk [Member]	Jun. 30, 2010 Knight Hawk [Member]	Jun. 30, 2010 Reserves [Member] Knight Hawk [Member]
Quarterly Financial Information [Line Items]
Revenues	$ 1,228,756,000	$ 1,198,673,000	$ 985,528,000	$ 872,938,000	$ 835,394,000	$ 874,705,000	$ 764,295,000	$ 711,874,000	$ 4,285,895,000	$ 3,186,268,000	$ 2,576,081,000
Gross profit	153,280,000	118,974,000	171,440,000	129,773,000	107,514,000	119,957,000	100,461,000	61,852,000
Income (loss) from operations	139,728,000	76,256,000	95,354,000	102,238,000	86,938,000	98,347,000	106,499,000	32,200,000	413,576,000	323,984,000	123,714,000	424,679,000	309,586,000	103,535,000
Net income	71,215,000	9,121,000	6,630,000	55,874,000	48,031,000	46,859,000	66,274,000	(1,770,000)	142,840,000	159,394,000	42,179,000	142,840,000	159,394,000	42,179,000
Basic earnings (loss) per common share	$ 0.34	$ 0.04	$ 0.04	$ 0.34	$ 0.29	$ 0.29	$ 0.41	$ (0.01)	$ 0.75	$ 0.98	$ 0.28
Diluted earnings (loss) per common share	$ 0.33	$ 0.04	$ 0.04	$ 0.34	$ 0.29	$ 0.29	$ 0.41	$ (0.01)	$ 0.74	$ 0.97	$ 0.28
Tons Of Coal															345,000,000						68,400,000
Additional ownership interest																			9.00%	9.00%
Gain on Knight Hawk transaction							41,600,000			41,577,000									41,600,000
Expensed costs related to acquisition																		98,200,000
Acquisition and transition costs									54,676,000							1,300,000	4,700,000
Quarterly Financial Information, Quarterly Charges and Credits, Amount Reconciling to Previously Reported Results		$ 10,200,000	$ 4,800,000
Effective tax rate (in hundredths)		37.00%	37.00%