Related Party Transactions (Financial Information for the Unconsolidated Mines) (Details) (Unconsolidated mines, NACoal, USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Unconsolidated mines | NACoal
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated mines and related tax assets	$ 28,200,000	$ 33,100,000
Maximum loss	4,000,000	5,400,000	3,200,000
Dividends from unconsolidated mines	38,300,000	35,200,000
Statement of Operations
Revenues	579,031,000	577,436,000	543,892,000
Gross profit	74,244,000	74,870,000	74,542,000
Income before income taxes	48,592,000	47,953,000	46,819,000
Net income	37,067,000	37,468,000	35,616,000
Balance Sheet
Current assets	143,105,000	147,370,000
Non-current assets	781,475,000	737,851,000
Current liabilities	177,659,000	148,264,000
Non-current liabilities	$ 742,938,000	$ 731,525,000