Earnings Per Share (CPE Inc. Only) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended			0 Months Ended		12 Months Ended												0 Months Ended	1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 15, 2010 CPE Inc	Nov. 19, 2009 CPE Inc	Dec. 31, 2012 CPE Inc	Dec. 31, 2011 CPE Inc	Dec. 31, 2010 CPE Inc	Dec. 31, 2010 CPE Inc CPE Resources common membership units held by noncontrolling interest	Dec. 31, 2012 CPE Inc Restricted Stock	Dec. 31, 2011 CPE Inc Restricted Stock	Dec. 31, 2010 CPE Inc Restricted Stock	Dec. 31, 2012 CPE Inc Options outstanding	Dec. 31, 2011 CPE Inc Options outstanding	Dec. 31, 2010 CPE Inc Options outstanding	Dec. 31, 2012 CPE Inc Employee stock purchase plan	Dec. 31, 2011 CPE Inc Employee stock purchase plan	Dec. 15, 2010 CPE Resources	Oct. 31, 2010 CPE Resources	Dec. 31, 2012 CPE Resources	Dec. 31, 2011 CPE Resources	Dec. 31, 2010 CPE Resources
Diluted earnings per share
Common stock issued to public (in shares)					30,600,000
Percentage of common membership units acquired to units outstanding					51.00%
Secondary Offering shares of common stock on behalf of Rio Tinto				29,400,000
Percentage of divestiture of Rio Tinto's holdings in LLC																			100.00%
Increase in number of weighted-average shares outstanding, in connection with vesting of shares issued to employees and directors										62,000
Shares retained as treasury shares	276	0				276,000
Common membership units acquired from RTEA				29,400,000														29,400,000
Combined federal and state statutory rate (as a percent)	36.00%					35.00%	35.00%	35.00%												35.00%	35.00%	35.00%
Numerator for calculation of diluted earnings per share:
Net income						$ 173,720	$ 189,797	$ 33,737
Denominator for basic income per share - weighted-average shares outstanding	60,093,000	60,004,000	31,889,000			60,093,000	60,004,000	31,889,000
Basic earnings per share (in dollars per share)	$ 2.89	$ 3.16	$ 1.06			$ 2.89	$ 3.16	$ 1.06
Dilutive effect of stock equivalents (in shares)						834,000	633,000
Denominator for diluted earnings per share (in shares)	60,927,000	60,637,000	31,889,000			60,927,000	60,637,000	31,889,000
Diluted earnings per share (in dollars per share)	$ 2.85	$ 3.13	$ 1.06			$ 2.85	$ 3.13	$ 1.06
Anti-dilutive securities excluded from diluted earnings per share calculation
Number of anti-dilutive securities excluded									25,695,000	28,000	7,000	878,000	57,000	73,000	1,032,000	21,000	8,000