Significant Acquisitions - Additional Information (Detail) (USD $)	9 Months Ended	6 Months Ended	9 Months Ended		9 Months Ended
	Sep. 30, 2012 acre	Jun. 30, 2012 Northern Oklahoma [Member] acre	Sep. 30, 2012 Sugar Camp [Member]	Mar. 31, 2012 Sugar Camp [Member]	Sep. 30, 2012 Ruger Llc [Member]	Sep. 30, 2012 Colt [Member]	Sep. 30, 2009 Colt [Member] T
Business Acquisition [Line Items]
Coal reserve agreed to acquire under partnership agreement							200,000,000
Purchase price				$ 50,000,000			$ 255,000,000
Amount paid at closing	80,000,000					40,000,000
Land acquired	15,600	19,200
Cost of partnership acquisition	51,300,000	63,900,000
Quarterly minimum payments	1,250,000
Total projected payments	97,100,000
Unearned income	47,600,000
Net amount receivable under the lease			50,000,000		7,300,000
Net amount receivable under the lease included in accounts receivable - affiliates			1,700,000
Transportation cost of equipment			2,600,000
Contractual overriding royalty interest				8,900,000
Amount expected to be received over the next twelve months					1,100,000
Contractual overriding royalty interest					$ 800,000