Supplemental Guarantor/Non-Guarantor Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues	$ 1,326.2	$ 1,027.2
Operating Costs and Expenses	963.7	920.2
Depreciation, Depletion and Amortization	119.9	111.8
Asset retirement obligation expenses	14.6	13.1
Selling and administrative expenses	37.2	48.3
Selling and Administrative expenses related to debt restructuring	0.0
Restructuring charges	0.0	12.1
Net gain on disposal of assets	22.8	1.8
Asset impairment		14.3
Asset impairment	30.5	17.2	$ 247.9	$ 1,277.8	$ 154.4
Loss from equity affiliates	15.0	(9.0)
Interest expense	32.9	126.2
Interest income	2.7	1.4
Reorganization Items, net	41.4	0.0
Income (Loss) from Continuing Operations before Income Taxes	126.5	(227.5)
Income tax provision (benefit)	2.2	(59.8)
Loss from continuing operations, net of income taxes	124.3	(167.7)
Loss from discontinued operations, net of income taxes	(4.1)	(3.4)
Net income (loss)	120.2	(171.1)
Net Income (Loss) Attributable to Noncontrolling Interest	4.8	0.0
Net Income (Loss) Attributable to Common Stockholder	115.4	(171.1)
Other Comprehensive Income (Loss), Net of Tax	28.5	56.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	148.7	(115.1)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	4.8	0.0
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	143.9	(115.1)
Parent Company [Member]
Revenues	0.0	0.0
Operating Costs and Expenses	27.5	17.9
Depreciation, Depletion and Amortization	0.0	0.0
Asset retirement obligation expenses	0.0	0.0
Selling and administrative expenses	2.0	2.4
Restructuring charges		0.0
Net gain on disposal of assets	0.0	0.0
Asset impairment	0.0
Asset impairment		0.0
Loss from equity affiliates	(170.5)	(59.3)
Interest expense	32.9	122.6
Interest income	0.0	0.0
Reorganization Items, net	(0.2)
Income (Loss) from Continuing Operations before Income Taxes	108.3	(202.2)
Income tax provision (benefit)	(10.0)	(33.1)
Loss from continuing operations, net of income taxes	118.3	(169.1)
Loss from discontinued operations, net of income taxes	(2.9)	(2.0)
Net income (loss)	115.4	(171.1)
Net Income (Loss) Attributable to Noncontrolling Interest	0.0
Net Income (Loss) Attributable to Common Stockholder	115.4	(171.1)
Other Comprehensive Income (Loss), Net of Tax	28.5	56.0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	143.9
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.0
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	143.9	(115.1)
Guarantor Subsidiaries [Member]
Revenues	732.9	663.9
Operating Costs and Expenses	551.4	522.3
Depreciation, Depletion and Amortization	69.7	51.1
Asset retirement obligation expenses	7.2	6.5
Selling and administrative expenses	30.6	42.2
Restructuring charges		11.4
Net gain on disposal of assets	(22.9)	1.8
Asset impairment	0.0
Asset impairment		0.0
Loss from equity affiliates	0.9	(1.2)
Interest expense	4.4	5.8
Interest income	(2.1)	1.0
Reorganization Items, net	41.0
Income (Loss) from Continuing Operations before Income Taxes	52.7	26.2
Income tax provision (benefit)	(2.7)	(18.5)
Loss from continuing operations, net of income taxes	55.4	44.7
Loss from discontinued operations, net of income taxes	(0.3)	(0.5)
Net income (loss)	55.1	44.2
Net Income (Loss) Attributable to Noncontrolling Interest	0.0
Net Income (Loss) Attributable to Common Stockholder	55.1	44.2
Other Comprehensive Income (Loss), Net of Tax	5.8	5.1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	60.9
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.0
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	60.9	49.3
Non-Guarantor Subsidiaries [Member]
Revenues	661.7	441.1
Operating Costs and Expenses	453.2	457.8
Depreciation, Depletion and Amortization	50.2	60.7
Asset retirement obligation expenses	7.4	6.6
Selling and administrative expenses	4.6	3.7
Restructuring charges		0.7
Net gain on disposal of assets	0.1	0.0
Asset impairment	30.5
Asset impairment		17.2
Loss from equity affiliates	(15.9)	7.8
Interest expense	5.5	6.2
Interest income	(10.5)	8.8
Reorganization Items, net	0.6
Income (Loss) from Continuing Operations before Income Taxes	136.0	(110.8)
Income tax provision (benefit)	14.9	(8.2)
Loss from continuing operations, net of income taxes	121.1	(102.6)
Loss from discontinued operations, net of income taxes	(0.9)	(0.9)
Net income (loss)	120.2	(103.5)
Net Income (Loss) Attributable to Noncontrolling Interest	4.8
Net Income (Loss) Attributable to Common Stockholder	115.4	(103.5)
Other Comprehensive Income (Loss), Net of Tax	5.5	2.7
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	125.7
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	4.8
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	120.9	(100.8)
Debtor Subsidiaries [Member]
Revenues	733.0
Operating Costs and Expenses	572.5
Depreciation, Depletion and Amortization	75.2
Asset retirement obligation expenses	9.4
Selling and administrative expenses	32.7
Net gain on disposal of assets	22.9
Asset impairment	30.5
Loss from equity affiliates	141.4
Interest expense	32.8
Interest income	2.5
Reorganization Items, net	40.8
Income (Loss) from Continuing Operations before Income Taxes	105.9
Income tax provision (benefit)	(12.7)
Loss from continuing operations, net of income taxes	118.6
Loss from discontinued operations, net of income taxes	(3.2)
Net income (loss)	115.4
Net Income (Loss) Attributable to Noncontrolling Interest	0.0
Net Income (Loss) Attributable to Common Stockholder	115.4
Non-Debtor Subsidiaries [Member]
Revenues	594.5
Operating Costs and Expenses	392.5
Depreciation, Depletion and Amortization	44.7
Asset retirement obligation expenses	5.2
Selling and administrative expenses	4.5
Net gain on disposal of assets	(0.1)
Asset impairment	0.0
Loss from equity affiliates	15.8
Interest expense	5.2
Interest income	5.3
Reorganization Items, net	0.6
Income (Loss) from Continuing Operations before Income Taxes	162.8
Income tax provision (benefit)	14.9
Loss from continuing operations, net of income taxes	147.9
Loss from discontinued operations, net of income taxes	(0.9)
Net income (loss)	147.0
Net Income (Loss) Attributable to Noncontrolling Interest	4.8
Net Income (Loss) Attributable to Common Stockholder	142.2
Consolidation, Eliminations [Member]
Revenues	(68.4)	(77.8)
Operating Costs and Expenses	(68.4)	(77.8)
Depreciation, Depletion and Amortization	0.0	0.0
Asset retirement obligation expenses	0.0	0.0
Selling and administrative expenses	0.0	0.0
Restructuring charges		0.0
Net gain on disposal of assets	0.0	0.0
Asset impairment	0.0
Asset impairment		0.0
Loss from equity affiliates	170.5	59.3
Interest expense	(9.9)	(8.4)
Interest income	9.9	(8.4)
Reorganization Items, net	0.0
Income (Loss) from Continuing Operations before Income Taxes	(170.5)	59.3
Income tax provision (benefit)	0.0	0.0
Loss from continuing operations, net of income taxes	(170.5)	59.3
Loss from discontinued operations, net of income taxes	0.0	0.0
Net income (loss)	(170.5)	59.3
Net Income (Loss) Attributable to Noncontrolling Interest	0.0
Net Income (Loss) Attributable to Common Stockholder	(170.5)	59.3
Other Comprehensive Income (Loss), Net of Tax	(11.3)	(7.8)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	(181.8)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0.0
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ (181.8)	$ 51.5