Other Events and Transactions (Sales Agreement and Development Activities) (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Long Lived Assets Held-for-sale [Line Items]
Indemnification receivable	$ 2,800,000
Cash in escrow for investment	5,000,000	0
Proceeds from the sale of assets	1,316,000	34,539,000
Accounts receivable from affiliates	30,551,000	3,242,000	28,144,000
NACoal
Long Lived Assets Held-for-sale [Line Items]
Number of draglines sold		2
Proceeds from the sale of assets		31,200,000
Number of draglines sold for a gain		1
Gain on sale of assets		3,300,000
Gain on sale of land		2,300,000
Coyote Creek
Long Lived Assets Held-for-sale [Line Items]
Accounts receivable from affiliates	26,700,000		24,800,000
Other Long-term Liabilities
Long Lived Assets Held-for-sale [Line Items]
Liabilities related to pre-acquisition obligations	3,100,000
Other Noncurrent Assets
Long Lived Assets Held-for-sale [Line Items]
Escrow assets	$ 5,000,000