Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
BWM
Related Party Transactions
Ownership interest in the joint venture accounted for under the proportionate consolidation method (as a percent)	50.00%
Charges to joint venture	$ 1.9	$ 2.4	$ 2.9
Belcourt Saxon
Related Party Transactions
Ownership interest in the joint venture accounted for under the proportionate consolidation method (as a percent)	50.00%
Number of multi-deposit coal properties owned by the related party	2
Belcourt Saxon | Minimum
Related Party Transactions
Distance of multi-deposit coal properties from south of Wolverine Mine	40
Belcourt Saxon | Maximum
Related Party Transactions
Distance of multi-deposit coal properties from south of Wolverine Mine	80