Supplemental Guarantor/Non-Guarantor Financial Information (Details) - USD ($) $ in Millions	3 Months Ended											9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Audited Supplemental Condensed Consolidated Statements of Operations
Total revenues	$ 1,440.8	$ 1,207.1	$ 1,040.2	$ 1,027.2	$ 1,313.1				$ 1,418.9	$ 1,339.3	$ 1,537.9	$ 3,564.7	$ 4,715.3	$ 5,609.2	$ 6,792.2
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)												3,035.9	4,107.6	5,007.7	5,716.9
Depreciation, depletion and amortization												336.7	465.4	572.2	655.7
Asset retirement obligation expenses												26.9	41.8	45.5	81.0
Selling and administrative expenses												90.7	153.4	176.4	227.1
Restructuring and pension settlement charges							$ 21.2						15.5	23.5	26.0
Other operating (income) loss:
Net gain on disposal of assets			(13.7)									21.4	(23.2)	(45.0)	(41.4)
Asset impairment	230.7			17.2	377.0		900.8					230.7	247.9	1,277.8	154.4
Loss from equity affiliates and investment in subsidiaries	(28.8)											26.2	(16.2)	15.9	107.6
Interest expense				126.2								150.4	298.6	465.4	426.6
Loss on early debt extinguishment	(29.5)						8.3	$ 59.5				(29.5)	29.5	67.8	1.6
Interest income												(4.0)	(5.7)	(7.7)	(15.4)
Reorganization Items	33.9	29.7	95.4									159.0	159.0	0.0	0.0
(Loss) income from continuing operations before income taxes												(446.3)	(758.3)	(1,990.3)	(547.9)
Income tax (benefit) provision					7.9		$ 67.4					(22.7)	(94.5)	(207.1)	147.4
(Loss) Income from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(175.2)	(97.7)	(223.2)	(167.7)	(502.0)				(126.4)	(1,006.0)	(148.8)	(423.6)	(663.8)	(1,783.2)	(695.3)
Loss from discontinued operations, net of income taxes	13.1	38.1				$ 155.1		$ 7.6				(53.6)	(57.6)	(175.0)	(28.2)
Net (loss) income	(188.3)	(135.8)	(226.2)	(171.1)	(474.3)				(283.9)	(1,042.3)	(157.7)	(477.2)	(721.4)	(1,958.2)	(723.5)
Less: Net income attributable to noncontrolling interests												7.9	7.9	7.1	9.7
Net (loss) income attributable to common stockholders	$ (192.7)	$ (137.6)	$ (227.9)	$ (171.1)	$ (473.5)				$ (286.7)	$ (1,044.1)	$ (161.0)	$ (485.1)	(729.3)	(1,965.3)	(733.2)
Parent Company [Member]
Audited Supplemental Condensed Consolidated Statements of Operations
Total revenues													0.0	0.0	0.0
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)													172.9	436.6	49.6
Depreciation, depletion and amortization													0.0	0.0	0.0
Asset retirement obligation expenses													0.0	0.0	0.0
Selling and administrative expenses													12.8	32.1	46.8
Restructuring and pension settlement charges													0.0	(3.9)	0.0
Other operating (income) loss:
Net gain on disposal of assets													0.0	(2.3)	0.0
Asset impairment													0.0	0.0	4.7
Loss from equity affiliates and investment in subsidiaries													174.5	903.2	74.7
Interest expense													288.6	468.4	423.1
Loss on early debt extinguishment													29.5	67.8	1.6
Interest income													(0.2)	(14.0)	(15.3)
Reorganization Items													73.4
(Loss) income from continuing operations before income taxes													(751.5)	(1,887.9)	(585.2)
Income tax (benefit) provision													(84.6)	(87.4)	116.4
(Loss) Income from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest													(666.9)	(1,800.5)	(701.6)
Loss from discontinued operations, net of income taxes													(62.4)	(164.8)	(31.6)
Net (loss) income													(729.3)	(1,965.3)	(733.2)
Less: Net income attributable to noncontrolling interests													0.0	0.0	0.0
Net (loss) income attributable to common stockholders													(729.3)	(1,965.3)	(733.2)
Guarantor Subsidiaries [Member]
Audited Supplemental Condensed Consolidated Statements of Operations
Total revenues													2,830.0	3,535.3	4,063.8
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)													2,172.4	2,782.6	3,121.9
Depreciation, depletion and amortization													217.4	249.7	271.0
Asset retirement obligation expenses													15.8	13.2	23.2
Selling and administrative expenses													126.5	132.6	161.1
Restructuring and pension settlement charges													11.9	11.4	26.0
Other operating (income) loss:
Net gain on disposal of assets													(21.4)	(29.8)	(17.7)
Asset impairment													37.5	308.6	63.3
Loss from equity affiliates and investment in subsidiaries													4.5	6.9	7.6
Interest expense													19.6	19.6	19.5
Loss on early debt extinguishment													0.0	0.0	0.0
Interest income													(4.8)	(2.4)	(2.9)
Reorganization Items													82.1
(Loss) income from continuing operations before income taxes													168.5	42.9	390.8
Income tax (benefit) provision													(11.0)	(108.2)	23.7
(Loss) Income from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest													179.5	151.1	367.1
Loss from discontinued operations, net of income taxes													(0.1)	1.6	(7.2)
Net (loss) income													179.4	152.7	359.9
Less: Net income attributable to noncontrolling interests													0.0	0.8	5.2
Net (loss) income attributable to common stockholders													179.4	151.9	354.7
Non-Guarantor Subsidiaries [Member]
Audited Supplemental Condensed Consolidated Statements of Operations
Total revenues													2,189.8	2,535.3	3,311.7
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)													2,066.8	2,249.9	3,128.7
Depreciation, depletion and amortization													248.0	322.5	384.7
Asset retirement obligation expenses													26.0	32.3	57.8
Selling and administrative expenses													14.1	11.7	19.2
Restructuring and pension settlement charges													3.6	16.0	0.0
Other operating (income) loss:
Net gain on disposal of assets													(1.8)	(12.9)	(23.7)
Asset impairment													210.4	969.2	86.4
Loss from equity affiliates and investment in subsidiaries													(20.7)	9.0	100.0
Interest expense													24.4	24.7	34.3
Loss on early debt extinguishment													0.0	0.0	0.0
Interest income													(34.7)	(38.6)	(47.5)
Reorganization Items													3.5
(Loss) income from continuing operations before income taxes													(349.8)	(1,048.5)	(428.2)
Income tax (benefit) provision													1.1	(11.5)	7.3
(Loss) Income from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest													(350.9)	(1,037.0)	(435.5)
Loss from discontinued operations, net of income taxes													4.9	(11.8)	10.6
Net (loss) income													(346.0)	(1,048.8)	(424.9)
Less: Net income attributable to noncontrolling interests													7.9	6.3	4.5
Net (loss) income attributable to common stockholders													(353.9)	(1,055.1)	(429.4)
Consolidation, Eliminations [Member]
Audited Supplemental Condensed Consolidated Statements of Operations
Total revenues													(304.5)	(461.4)	(583.3)
Costs and expenses
Operating costs and expenses (exclusive of items shown separately below)													(304.5)	(461.4)	(583.3)
Depreciation, depletion and amortization													0.0	0.0	0.0
Asset retirement obligation expenses													0.0	0.0	0.0
Selling and administrative expenses													0.0	0.0	0.0
Restructuring and pension settlement charges													0.0	0.0	0.0
Other operating (income) loss:
Net gain on disposal of assets													0.0	0.0	0.0
Asset impairment													0.0	0.0	0.0
Loss from equity affiliates and investment in subsidiaries													(174.5)	(903.2)	(74.7)
Interest expense													(34.0)	(47.3)	(50.3)
Loss on early debt extinguishment													0.0	0.0	0.0
Interest income													34.0	47.3	50.3
Reorganization Items													0.0
(Loss) income from continuing operations before income taxes													174.5	903.2	74.7
Income tax (benefit) provision													0.0	0.0	0.0
(Loss) Income from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest													174.5	903.2	74.7
Loss from discontinued operations, net of income taxes													0.0	0.0	0.0
Net (loss) income													174.5	903.2	74.7
Less: Net income attributable to noncontrolling interests													0.0	0.0	0.0
Net (loss) income attributable to common stockholders													$ 174.5	$ 903.2	$ 74.7