Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statutory Accounting Practices
Mineral rights	$ 338.1	$ 361.2
Deferred finance costs, net	4.7	5.7
Deferred finance costs, current	$ 2.2	$ 2.6
Exploration and production equipment | Minimum
Statutory Accounting Practices
Estimated useful life	1 year
Exploration and production equipment | Maximum
Statutory Accounting Practices
Estimated useful life	18 years
Building and building improvements | Minimum
Statutory Accounting Practices
Estimated useful life	1 year
Building and building improvements | Maximum
Statutory Accounting Practices
Estimated useful life	20 years
Mining properties and mineral rights | Minimum
Statutory Accounting Practices
Term of contract	10 years
Mining properties and mineral rights | Maximum
Statutory Accounting Practices
Term of contract	50 years
Accounting Standards Update 2016-02 | Minimum
Statutory Accounting Practices
RIght-of-use assets and lease liabilities	$ 15.0
Lease liability	15.0
Accounting Standards Update 2016-02 | Maximum
Statutory Accounting Practices
RIght-of-use assets and lease liabilities	20.0
Lease liability	$ 20.0