ACQUISITION OF BUSINESS (Details 2) (USD $)	3 Months Ended											12 Months Ended			1 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 31, 2011 Green River	Dec. 31, 2012 Green River	Dec. 31, 2012 Green River	Dec. 31, 2011 Green River
Business Acquisition
Total revenues, As reported	$ 549,318,000	[1]	$ 511,348,000	[2]	$ 529,772,000	$ 443,492,000	$ 474,519,000	[1],[3]	$ 487,657,000	$ 457,855,000	$ 423,172,000	$ 2,033,930,000	$ 1,843,203,000	$ 1,609,743,000			$ 2,033,930,000	$ 1,843,203,000
Total revenues, Pro forma																	2,061,273,000	1,957,241,000
Net income, as reported	96,126,000	[1]	58,733,000	[2]	93,881,000	82,486,000	90,949,000	[1],[3]	103,667,000	97,286,000	94,441,000	331,226,000	386,343,000	317,301,000			331,226,000	386,343,000
Net income, Pro forma																	332,507,000	397,717,000
Revenue of acquired business reflected in consolidated statements of income																81,600,000	81,600,000
Net income of acquired business reflected in consolidated statements of income																7,600,000	7,600,000
Acquisition costs															$ 600,000

[1] The comparability of our December 31, 2012 and 2011 quarterly results were affected by a $14.0 million and $13.6 million, respectively, decrease in the ARLP Partnership's workers' compensation liability, excluding discount rate changes, due to the completion of its annual actuarial study, which reflected a favorable development in the ARLP Partnership's disability emergence patterns and claims estimates, as well as improved visibility of the Mettiki (WV) claims experience.

[2] During the quarter ended September 30, 2012, the ARLP Partnership recorded a $19.0 million impairment of the carrying value of assets at the Pontiki mine (Note 5).

[3] During the quarter ended December 31, 2011, the ARLP Partnership corrected the interest rate used to account for capitalized interest to utilize the Partnership's overall borrowing rate. This correction resulted in a $10.0 million increase in property, plant and equipment (net of $1.2 million of related depreciation, depletion and amortization) and an $11.2 million offsetting decrease to interest expense ($9.4 million of which was related to prior years). Management concluded the effect of the correction was not material to prior periods, 2011 results or the trend of earnings.