Asset Impairment and Mine Closure Costs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures						$ 26.9
Acceleration of asset retirement obligations						7.1
Employee termination benefits						6.7
Other charges related to mine closures						4.3
Long-lived assets				149.7	463.6	806.7
Equity method investment					43.2
Marketable securities				4.7	21.5	35.5
Cost method investments						39.4
Other asset impairment charges						2.4
Total asset impairment and mine closure costs	154.4	506.8	21.5	154.4	528.3	929.0
Asset realization risk	124.1			124.1
Additional asset impairment charge deemed necessary for assets at risk				0
Australian Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures						0
Acceleration of asset retirement obligations						0
Employee termination benefits						0
Other charges related to mine closures						0
Long-lived assets				78.6	390.8	806.7
Equity method investment					0
Marketable securities				0	0	0
Cost method investments						0
Other asset impairment charges						0
Total asset impairment and mine closure costs				78.6	390.8	806.7
Western U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures						0
Acceleration of asset retirement obligations						0
Employee termination benefits						0
Other charges related to mine closures						0
Long-lived assets				2.7		0
Marketable securities				0		0
Cost method investments						0
Other asset impairment charges						2.4
Total asset impairment and mine closure costs				2.7		2.4
Midwestern U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures						26.9
Acceleration of asset retirement obligations						7.1
Employee termination benefits						6.7
Other charges related to mine closures						4.3
Long-lived assets						0
Marketable securities						0
Cost method investments						0
Other asset impairment charges						0
Total asset impairment and mine closure costs						45.0
Corporate and Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets related to mine closures						0
Acceleration of asset retirement obligations						0
Employee termination benefits						0
Other charges related to mine closures						0
Long-lived assets				68.4	72.8	0
Equity method investment					43.2
Marketable securities				4.7	21.5	35.5
Cost method investments						39.4
Other asset impairment charges						0
Total asset impairment and mine closure costs				73.1	137.5	74.9
Willow Lake Mine [Member] | Midwestern U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Total asset impairment and mine closure costs							45.0
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%