Asset Impairment and Mine Closure Costs (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets						$ 26.9
Acceleration of asset retirement obligations						7.1
Employee termination benefits						6.7
Other						4.3
Long-lived assets					463.6	806.7
Equity method investment					43.2
Marketable securities					21.5	35.5	0
Cost method investments						39.4
Other						2.4
Asset impairment charges	506.8	21.5	921.3	7.7	528.3	929.0	0
Australian Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets						0
Acceleration of asset retirement obligations						0
Employee termination benefits						0
Other						0
Long-lived assets					390.8	806.7
Equity method investment					0
Marketable securities					0	0
Cost method investments						0
Other						0
Asset impairment charges					390.8
Western U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets						0
Acceleration of asset retirement obligations						0
Employee termination benefits						0
Other						0
Long-lived assets						0
Marketable securities						0
Cost method investments						0
Other						2.4
Asset impairment charges						2.4
Midwestern U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets						26.9
Acceleration of asset retirement obligations						7.1
Employee termination benefits						6.7
Other						4.3
Long-lived assets						0
Marketable securities						0
Cost method investments						0
Other						0
Asset impairment charges						45.0
Corporate and Other [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of long-lived assets						0
Acceleration of asset retirement obligations						0
Employee termination benefits						0
Other						0
Long-lived assets					72.8	0
Equity method investment					43.2
Marketable securities			35.5		21.5
Cost method investments						39.4
Other						0
Asset impairment charges					137.5	74.9
Willow Lake Mine [Member] | Midwestern U.S. Mining [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges			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%