Fair Value Disclosure (Narrative) (Details) - Bellaire Corporation - Level 1 - Fair value measurements, recurring $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
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	$ 0.2	$ 0.1

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).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration