Leases (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating Lease, Right-of-Use Asset	$ 17.1
Operating lease expense	8.6	$ 12.0	$ 12.4
Royalty expense	103.7	$ 149.5	$ 166.1
Surety bonds outstanding	$ 31.7
Minimum
Lessee, Lease, Description [Line Items]
Operating lease, remaining lease terms	1 year
Maximum
Lessee, Lease, Description [Line Items]
Operating lease, remaining lease terms	7 years