Investments (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 65.3us-gaap_EquityMethodInvestments	$ 180.1us-gaap_EquityMethodInvestments
Loss from equity affiliates	107.6	40.2	61.2
Other-than-temporary impairment losses on equity method investments		43.2
Middlemount Mine [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Revenue	165	157	118
Change in deferred tax asset valuation allowance related to equity affiliates	52.3
Equity Method Investment, Summarized Financial Information, Current Assets	27.8	30.9
Equity Method Investment, Summarized Financial Information, Noncurrent Assets	424.4	525.3
Equity Method Investment, Summarized Financial Information, Current Liabilities	382.9	365.6
Equity Method Investment, Summarized Financial Information, Noncurrent Liabilities	11.3	17.2
Interest In Middlemount Coal Pty Limited [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	58.0	173.4
Loss from equity affiliates	98.5	33.5	52.1
Equity method investment, ownership percentage	50.00%
Other-than-temporary impairment losses on equity method investments		43.2
Remaining unamortized difference	96.1
Other Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	7.3	6.7
Loss from equity affiliates	$ 9.1	$ 6.7	$ 9.1
Middlemount Mine [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	50.00%	50.00%

Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee.

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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The amount of current assets reported by an equity method investment of the entity.

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The amount of noncurrent assets reported by an equity method investment of the entity.

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The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity.

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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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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.

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