Asset Impairment and Mine Closure Costs (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures						$ 463.6
Acceleration of asset retirement obligations						43.2
Employee termination benefits						21.5
Long-lived assets				$ 1,001.3	$ 149.7
Marketable securities				0.0	4.7	21.5
Equity method investment				276.5		43.2
Asset Impairment	$ 377.0	$ 900.8	$ 154.4	1,277.8	154.4	528.3
Asset realization risk	186.1			186.1
Additional asset impairment charge deemed necessary for assets at risk				0.0
Australian Metallurgical Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Long-lived assets					66.7	390.8
Australian Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures						390.8
Acceleration of asset retirement obligations						0.0
Employee termination benefits						0.0
Long-lived assets	144.5	$ 230.5		675.2	66.7
Marketable securities					0.0
Equity method investment				0.0
Asset Impairment				675.2	66.7	390.8
Australian Thermal Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Long-lived assets				17.5	11.9
Marketable securities					0.0
Equity method investment				0.0
Asset Impairment				17.5	11.9
Western U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Long-lived assets					2.7
Marketable securities					0.0
Asset Impairment					2.7
Midwestern U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Long-lived assets	9.7			40.2
Equity method investment				0.0
Asset Impairment				40.2
Corporate and Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures				182.2		72.8
Acceleration of asset retirement obligations						43.2
Employee termination benefits						21.5
Long-lived assets	86.2			268.4	68.4
Marketable securities					4.7	21.5
Equity method investment				276.5
Asset Impairment				544.9	$ 73.1	137.5
Remediation Property for Sale, Abandonment or Disposal [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Long-lived assets	$ 317.7
Midwestern Inactive Surface Mine [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Long-lived assets				$ 30.5
Eastern US Coal Reserves [Member] | Corporate and Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Long-lived assets						66.3
Third Party Mined Coal Reserves [Member] | Corporate and Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Long-lived assets						$ 6.5
Middlemount Mine [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Equity method investment, ownership percentage	50.00%		50.00%	50.00%	50.00%
Subordinated Borrowing, Name [Domain]
Impaired Long-Lived Assets Held and Used [Line Items]
Equity method investment				$ 229.9
Financing Receivable [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Investments and Other Noncurrent Assets	$ 65.2		$ 347.2	$ 65.2	$ 347.2
Middlemount Mine [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Equity method investment, ownership percentage	50.00%			50.00%
Middlemount Mine [Member] | Financing Receivable [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Investments and Other Noncurrent Assets	$ 65.2		$ 319.6	$ 65.2	$ 319.6
Middlemount Mine [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Equity method investment				$ 46.6