Related Party Transactions (Financial Information for the Unconsolidated Mines) (Details) (NACoal, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Equity Method Investments [Line Items]
Number of variable interest entities	10
Unconsolidated mines
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated mines and related tax assets	$ 20.2	$ 22.0
Maximum loss	3.2	6.3	5.0
Dividends from unconsolidated mines	38.8	35.2
Statement of Operations
Revenues	543.9	502.6	461.7
Gross profit	74.5	71.7	71.7
Income before income taxes	46.8	47.0	43.4
Income from continuing operations	35.6	36.5	33.1
Net income	35.6	36.5	33.1
Balance Sheet
Current assets	148.6	144.1
Non-current assets	836.3	685.2
Current liabilities	144.3	162.1
Non-current liabilities	$ 837.4	$ 660.9