Operations and Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Residual Value Guarantee, Description		Our largest operating lease is for the dragline at BNI Energy which includes a termination payment at the end of the lease term if we do not exercise our purchase option.
Operating Lease, Right-of-Use Asset	[1]	$ 28.6	$ 0.0
Operating Lease, Cost		9.4
Operating Lease, Payments		9.4
Operating Lease, Liability, Current	[2]	6.9	0.0
Operating Lease, Liability, Noncurrent	[3]	21.8	$ 0.0
Operating Lease, Liability		28.7
Residual Value of Leased Asset		$ 3.0
Vehicles and Equipment [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Weighted Average Remaining Lease Term		4 years
Operating Lease, Weighted Average Discount Rate, Percent		3.70%
Land and Other [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Weighted Average Remaining Lease Term		28 years
Operating Lease, Weighted Average Discount Rate, Percent		4.10%

[1] See Leases.

[2] See Leases.

[3] See Leases.