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Investments (Details 2) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Marketable securities |  |  | $ 0.0 | $ 4.7 | $ 4.7
Equity method investments | $ 0.5 | $ 0.5 | 0.5 | 1.5 |
(Gain) loss from equity affiliates | (28.8) | 26.2 | (16.2) | 15.9 | 107.6
Other-than-temporary impairment losses on equity method investments |  |  |  | 276.5 |
Middlemount Mine [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity Method Investment, Summarized Financial Information, Revenue |  |  | 183.0 | 160.0 | 165.0
Change in deferred tax asset valuation allowance related to equity affiliates |  |  |  |  | (52.3)
Other-than-temporary impairment losses on equity method investments |  |  |  | 46.6 |
Equity Method Investment, Summarized Financial Information, Current Assets | 47.3 | 47.3 | 47.3 | 31.7 |
Equity Method Investment, Summarized Financial Information, Noncurrent Assets | 263.4 | 263.4 | 263.4 | 348.0 |
Equity Method Investment, Summarized Financial Information, Current Liabilities | 363.5 | 363.5 | 363.5 | 362.2 |
Equity Method Investment, Summarized Financial Information, Noncurrent Liabilities | 50.3 | 50.3 | 50.3 | 10.5 |
Interest In Middlemount Coal Pty Limited [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity method investments | $ 0.0 | $ 0.0 | 0.0 | 0.0 |
(Gain) loss from equity affiliates |  |  | $ (22.6) | 7.0 | 98.5
Equity method investment, ownership percentage | 50.00% | 50.00% | 50.00% |  |
Other-than-temporary impairment losses on equity method investments |  |  |  | 276.5 |
Subordinated Borrowing, Name [Domain] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Other-than-temporary impairment losses on equity method investments |  |  | $ 229.9 | 229.9 |
Other Equity Method Investments [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity method investments | $ 0.5 | $ 0.5 | 0.5 | 1.5 |
(Gain) loss from equity affiliates |  |  | $ 6.4 | 8.9 | 9.1
Middlemount Mine [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity method investment, ownership percentage | 50.00% | 50.00% | 50.00% |  |
Corporate and Other [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Marketable securities |  |  |  |  | 4.7
(Gain) loss from equity affiliates |  |  | $ (16.2) | 15.9 | $ 107.6
Other-than-temporary impairment losses on equity method investments |  |  |  | 276.5 |
Financing Receivable [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Investments and Other Noncurrent Assets | $ 84.8 | $ 84.8 | $ 84.8 | 65.2 |
Middlemount Mine [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Equity method investment, ownership percentage | 50.00% | 50.00% | 50.00% |  |
Middlemount Mine [Member] | Financing Receivable [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Investments and Other Noncurrent Assets | $ 84.8 | $ 84.8 | $ 84.8 | $ 65.2 |

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