Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Operating segment results
Revenues	$ 1,758.0	$ 1,725.3	$ 3,384.8	$ 3,473.3
Adjusted EBITDA	213.1	254.3	390.0	534.4
Reconciliation Of Adjusted EBITDA To Consolidated (Loss) Income From Continuing Operations, Net of Income Taxes [Abstract]
Depreciation, depletion and amortization	(163.1)	(185.7)	(320.3)	(356.4)
Asset impairment	0	(21.5)	0	(21.5)
Amortization of basis difference related to equity affiliates	(1.3)	(2.4)	(2.5)	(4.0)
Asset retirement obligation expenses	(15.9)	(18.3)	(31.5)	(37.3)
Interest expense	(105.2)	(110.8)	(208.5)	(212.1)
Interest income	4.4	1.1	8.0	7.0
Income tax (provision) benefit	(4.0)	184.7	48.5	181.0
(Loss) income from continuing operations, net of income taxes	(72.0)	101.4	(116.3)	91.1
Australian Mining [Member]
Operating segment results
Revenues	707.4	744.8	1,319.2	1,482.8
Adjusted EBITDA	12.2	112.5	14.0	212.9
Western U.S. Mining [Member]
Operating segment results
Revenues	726.6	640.9	1,408.6	1,281.0
Adjusted EBITDA	222.0	154.9	395.9	315.4
Midwestern U.S. Mining [Member]
Operating segment results
Revenues	304.6	330.0	607.6	666.7
Adjusted EBITDA	69.9	106.8	148.6	219.1
Trading and Brokerage [Member]
Operating segment results
Revenues	10.3	(3.4)	31.3	22.5
Adjusted EBITDA	6.3	(41.0)	4.4	(25.0)
Corporate and Other [Member]
Operating segment results
Revenues	9.1	13.0	18.1	20.3
Adjusted EBITDA	$ (97.3)	$ (78.9)	$ (172.9)	$ (188.0)