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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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