Commitments, Guarantees and Contingencies - Leasing Agreements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Leasing Agreements [Line Items]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 11.5
Operating Leases, Future Minimum Payments, Due in Two Years	11.7
Operating Leases, Future Minimum Payments, Due in Three Years	11.4
Operating Leases, Future Minimum Payments, Due in Four Years	9.3
Operating Leases, Future Minimum Payments, Due in Five Years	8.5
Operating Leases, Future Minimum Payments, Due Thereafter	35.0
BNI Coal Dragline Lease [Member]
Leasing Agreements [Line Items]
Termination Fee	3.0
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 2.8