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Investments (Details 1) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items] |  |  |
Proceeds from sale of available-for-sale securities |  | $ 90.3 | $ 13.5
Net gains on sales and maturities |  | 0.0 | 0.0
Proceeds from Sale and Maturity of Marketable Securities | $ 0.0 | 90.3 | 13.5
Other-than-temporary impairment losses on marketable securities | $ 0.0 | $ 4.7 | 4.7
Middlemount Mine [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Valuation Allowances and Reserves, Balance |  |  | $ 52.3

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- Definition This element represents the amount of loss recognized for other than temporary impairments (OTTI) of investments in debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.M) -URI http://asc.fasb.org/extlink&oid=27011274&loc=d3e87990-122713 + Details
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- Definition Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 09 -Article 12 + Details
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