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Revenue Recognition (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Revenue from Contract with Customer [Abstract]        
Description of timing     The fixed portion of the transaction price will be recognized over the primary term of the contract, which is generally five years. The timing of the payment of the fixed portion of the transaction price is upfront, however, the performance obligation is satisfied over the primary term of the contract, which is generally five years. This revenue consists of up-front lease bonus payments received under royalty contracts that are recognized over the primary term of the royalty agreement, which is generally five years.  
Revenue recognized in contract liability $ 0.2 $ 0.3 $ 0.4 $ 0.6