Commitments, Guarantees and Contingencies - Leasing Agreements (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
Leasing Agreements [Line Items]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 10.9
Operating Leases, Future Minimum Payments, Due in Two Years	11.1
Operating Leases, Future Minimum Payments, Due in Three Years	11.4
Operating Leases, Future Minimum Payments, Due in Four Years	11.2
Operating Leases, Future Minimum Payments, Due in Five Years	9.2
Operating Leases, Future Minimum Payments, Due Thereafter	43.0
BNI Coal Dragline Lease
Leasing Agreements [Line Items]
Annual Lease Payment	2.8
Termination Fee	$ 3