Capital Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended		0 Months Ended		12 Months Ended	0 Months Ended	6 Months Ended
	Dec. 31, 2011	Dec. 31, 2009	Jul. 08, 2011 ICG [Member]	Jun. 08, 2011 ICG [Member]	Dec. 31, 2011 ICG [Member]	Aug. 06, 2009 Jacobs Ranch Mining Operations [Member]	Jul. 31, 2009 Jacobs Ranch Mining Operations [Member]	Jul. 31, 2011 Jacobs Ranch Mining Operations [Member]
Common stock issued	48,705,000	19,550,000	700,000	48,000,000		2,550,000	17,000,000
Common stock public offering price				$ 27.00				$ 17.50
Proceeds from the sale of common stock	$ 1,267,933	$ 326,452	$ 18,400		$ 1,250,000	$ 326,500
Stock repurchase authorized number	14,000,000
Outstanding shares available for repurchase	10,925,800