Commitments, Guarantees and Contingencies - Leasing Agreements (Details) $ in Millions	Jun. 30, 2015 USD ($)
Leasing Agreements [Line Items]
Operating Leases, Minimum Payments Due in 2015	$ 15.0
Operating Leases, Minimum Payments Due in 2016	12.9
Operating Leases, Minimum Payments Due in 2017	11.8
Operating Leases, Minimum Payments Due in 2018	10.4
Operating Leases, Minimum Payments Due in 2019	9.3
Operating Leases, Minimum Payments Due Thereafter	29.1
BNI Coal Dragline Lease [Member]
Leasing Agreements [Line Items]
Operating Leases, Minimum Payments Due in 2015	2.8
Operating Leases, Minimum Payments Due in 2016	2.8
Operating Leases, Minimum Payments Due in 2017	2.8
Operating Leases, Minimum Payments Due in 2018	2.8
Operating Leases, Minimum Payments Due in 2019	2.8
Operating Leases, Minimum Payments Due Thereafter	22.4
Termination Fee	$ 3.0