Asset Impairment and Mine Closure Costs (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended	12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2016
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of Long-Lived Assets to be Disposed of (after tax)	$ 193.2
Long-lived assets						$ 247.9	$ 1,001.3	$ 149.7
Marketable securities						0.0	4.7	4.7
Equity method investment							276.5
Asset Impairment	230.7	$ 17.2	$ 377.0	$ 900.8	$ 230.7	247.9	1,277.8	154.4
Asset realization risk	1,407.3				$ 1,407.3	1,407.3
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
Australian Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures						17.2
Long-lived assets	144.5			$ 230.5		193.2	675.2	66.7
Marketable securities								0.0
Equity method investment							0.0
Asset Impairment							675.2	66.7
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					37.5	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
Long-lived assets	86.2					54.7	268.4	68.4
Marketable securities								4.7
Equity method investment							276.5
Asset Impairment							544.9	$ 73.1
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
Middlemount Mine [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Equity method investment, ownership percentage	50.00%				50.00%	50.00%
Subordinated Borrowing, Name [Domain]
Impaired Long-Lived Assets Held and Used [Line Items]
Equity method investment						$ 229.9	229.9
Financing Receivable [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Investments and Other Noncurrent Assets	$ 84.8		65.2		$ 84.8	$ 84.8	65.2
Middlemount Mine [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Equity method investment, ownership percentage	50.00%				50.00%	50.00%
Middlemount Mine [Member] | Financing Receivable [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Investments and Other Noncurrent Assets	$ 84.8		$ 65.2		$ 84.8	$ 84.8	$ 65.2
Middlemount Mine [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Equity method investment						$ 46.6
Metropolitan mine [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Cash consideration for asset held for sale									$ 200.0