Supplemental Guarantor/Non-Guarantor Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Total revenues	$ 1,418.9	$ 1,722.9	$ 4,296.1	$ 5,107.7
Operating costs and expenses (exclusive of items shown separately below)	1,254.0	1,453.3	3,774.4	4,315.7
Depreciation, depletion and amortization	136.0	163.6	430.6	483.9
Asset retirement obligation expenses	12.3	15.0	40.4	46.5
Selling and administrative expenses	37.8	52.9	128.8	171.6
Restructuring charges	1.8	0.0	23.0	0.0
Other operating (income) loss:
Net (gain) loss on disposal of assets	(7.9)	(13.9)	(20.2)	(25.9)
Asset impairment	0.0	0.0	900.8	0.0
Loss (income) from equity affiliates and investment in subsidiaries	5.3	15.8	12.3	44.0
Interest expense	118.5	114.5	344.0	321.4
Loss on early debt extinguishment	0.0	0.0	67.8	1.6
Interest income	(1.4)	(3.7)	(6.6)	(11.7)
(Loss) income from continuing operations before income taxes	(137.5)	(74.6)	(1,399.2)	(239.4)
Income tax provision (benefit)	6.9	79.4	(83.2)	30.9
(Loss) income from continuing operations, net of income taxes	(144.4)	(154.0)	(1,316.0)	(270.3)
(Loss) income from discontinued operations, net of income taxes	(157.5)	5.0	(202.7)	6.0
Net (loss) income	(301.9)	(149.0)	(1,518.7)	(264.3)
Less: Net income attributable to noncontrolling interests	2.8	1.6	7.9	8.1
Net (loss) income attributable to common stockholders	(304.7)	(150.6)	(1,526.6)	(272.4)
Other comprehensive income (loss), net of income taxes	(51.0)	(207.6)	69.0	(7.2)
Comprehensive (loss) income	(352.9)	(356.6)	(1,449.7)	(271.5)
Less: Comprehensive income attributable to noncontrolling interests	2.8	1.6	7.9	8.1
Comprehensive (loss) income attributable to common stockholders	(355.7)	(358.2)	(1,457.6)	(279.6)
Parent Company [Member]
Total revenues	0.0	0.0	0.0	0.0
Operating costs and expenses (exclusive of items shown separately below)	116.8	(6.7)	326.9	7.2
Depreciation, depletion and amortization	0.0	0.0	0.0	0.0
Asset retirement obligation expenses	0.0	0.0	0.0	0.0
Selling and administrative expenses	6.5	12.0	24.3	36.6
Restructuring charges	0.0		(3.9)
Other operating (income) loss:
Net (gain) loss on disposal of assets	0.1	0.0	(2.3)	0.0
Asset impairment			0.0
Loss (income) from equity affiliates and investment in subsidiaries	(100.7)	80.9	594.9	45.7
Interest expense	120.4	106.0	348.3	317.3
Loss on early debt extinguishment			67.8	1.6
Interest income	(0.1)	0.0	(0.5)	(0.2)
(Loss) income from continuing operations before income taxes	(143.0)	(192.2)	(1,355.5)	(408.2)
Income tax provision (benefit)	4.6	(41.1)	(20.6)	(134.1)
(Loss) income from continuing operations, net of income taxes	(147.6)	(151.1)	(1,334.9)	(274.1)
(Loss) income from discontinued operations, net of income taxes	(157.1)	0.5	(191.7)	1.7
Net (loss) income	(304.7)	(150.6)	(1,526.6)	(272.4)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net (loss) income attributable to common stockholders	(304.7)	(150.6)	(1,526.6)	(272.4)
Other comprehensive income (loss), net of income taxes	(51.0)	(207.6)	69.0	(7.2)
Comprehensive (loss) income	(355.7)	(358.2)	(1,457.6)	(279.6)
Less: Comprehensive income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive (loss) income attributable to common stockholders	(355.7)	(358.2)	(1,457.6)	(279.6)
Guarantor Subsidiaries [Member]
Total revenues	877.2	1,008.1	2,627.1	2,989.5
Operating costs and expenses (exclusive of items shown separately below)	667.0	758.2	1,978.5	2,226.2
Depreciation, depletion and amortization	69.6	77.0	213.1	230.4
Asset retirement obligation expenses	5.2	6.5	17.2	22.1
Selling and administrative expenses	28.6	36.4	95.3	120.5
Restructuring charges	0.8		11.0
Other operating (income) loss:
Net (gain) loss on disposal of assets	(6.6)	(4.4)	(16.5)	(16.3)
Asset impairment			182.2
Loss (income) from equity affiliates and investment in subsidiaries	1.8	3.6	4.7	6.3
Interest expense	1.6	1.7	5.0	5.0
Loss on early debt extinguishment			0.0	0.0
Interest income	(2.8)	(2.8)	(8.6)	(7.3)
(Loss) income from continuing operations before income taxes	112.0	131.9	145.2	402.6
Income tax provision (benefit)	(114.9)	57.0	(177.2)	122.8
(Loss) income from continuing operations, net of income taxes	226.9	74.9	322.4	279.8
(Loss) income from discontinued operations, net of income taxes	(0.3)	(0.1)	(2.2)	(1.3)
Net (loss) income	226.6	74.8	320.2	278.5
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net (loss) income attributable to common stockholders	226.6	74.8	320.2	278.5
Other comprehensive income (loss), net of income taxes	12.0	2.7	30.3	5.1
Comprehensive (loss) income	238.6	77.5	350.5	283.6
Less: Comprehensive income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive (loss) income attributable to common stockholders	238.6	77.5	350.5	283.6
Non-Guarantor Subsidiaries [Member]
Total revenues	555.9	731.1	1,708.7	2,177.3
Operating costs and expenses (exclusive of items shown separately below)	484.4	718.1	1,508.7	2,141.4
Depreciation, depletion and amortization	66.4	86.6	217.5	253.5
Asset retirement obligation expenses	7.1	8.5	23.2	24.4
Selling and administrative expenses	2.7	4.5	9.2	14.5
Restructuring charges	1.0		15.9
Other operating (income) loss:
Net (gain) loss on disposal of assets	(1.4)	(9.5)	(1.4)	(9.6)
Asset impairment			718.6
Loss (income) from equity affiliates and investment in subsidiaries	3.5	12.2	7.6	37.7
Interest expense	2.3	12.3	7.9	15.4
Loss on early debt extinguishment			0.0	0.0
Interest income	(4.3)	(6.4)	(14.7)	(20.5)
(Loss) income from continuing operations before income taxes	(5.8)	(95.2)	(783.8)	(279.5)
Income tax provision (benefit)	117.2	63.5	114.6	42.2
(Loss) income from continuing operations, net of income taxes	(123.0)	(158.7)	(898.4)	(321.7)
(Loss) income from discontinued operations, net of income taxes	(0.1)	4.6	(8.8)	5.6
Net (loss) income	(123.1)	(154.1)	(907.2)	(316.1)
Less: Net income attributable to noncontrolling interests	2.8	1.6	7.9	8.1
Net (loss) income attributable to common stockholders	(125.9)	(155.7)	(915.1)	(324.2)
Other comprehensive income (loss), net of income taxes	(26.8)	(6.7)	(66.9)	(22.6)
Comprehensive (loss) income	(149.9)	(160.8)	(974.1)	(338.7)
Less: Comprehensive income attributable to noncontrolling interests	2.8	1.6	7.9	8.1
Comprehensive (loss) income attributable to common stockholders	(152.7)	(162.4)	(982.0)	(346.8)
Eliminations [Member]
Total revenues	(14.2)	(16.3)	(39.7)	(59.1)
Operating costs and expenses (exclusive of items shown separately below)	(14.2)	(16.3)	(39.7)	(59.1)
Depreciation, depletion and amortization	0.0	0.0	0.0	0.0
Asset retirement obligation expenses	0.0	0.0	0.0	0.0
Selling and administrative expenses	0.0	0.0	0.0	0.0
Restructuring charges	0.0		0.0
Other operating (income) loss:
Net (gain) loss on disposal of assets	0.0	0.0	0.0	0.0
Asset impairment			0.0
Loss (income) from equity affiliates and investment in subsidiaries	100.7	(80.9)	(594.9)	(45.7)
Interest expense	(5.8)	(5.5)	(17.2)	(16.3)
Loss on early debt extinguishment			0.0	0.0
Interest income	5.8	5.5	17.2	16.3
(Loss) income from continuing operations before income taxes	(100.7)	80.9	594.9	45.7
Income tax provision (benefit)	0.0	0.0	0.0	0.0
(Loss) income from continuing operations, net of income taxes	(100.7)	80.9	594.9	45.7
(Loss) income from discontinued operations, net of income taxes	0.0	0.0	0.0	0.0
Net (loss) income	(100.7)	80.9	594.9	45.7
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net (loss) income attributable to common stockholders	(100.7)	80.9	594.9	45.7
Other comprehensive income (loss), net of income taxes	14.8	4.0	36.6	17.5
Comprehensive (loss) income	(85.9)	84.9	631.5	63.2
Less: Comprehensive income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive (loss) income attributable to common stockholders	$ (85.9)	$ 84.9	$ 631.5	$ 63.2