Fair Value Disclosure (Narrative) (Details) - Bellaire Corporation - Level 1 - Fair value measurements, recurring $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mine Water Treatment Trust	$ 5.0	$ 5.0
Realized gain on the Mine Water Treatment Trust	$ 0.4	$ 0.5

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
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-Publisher FASB
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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
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