Income Taxes (Details 3) - USD ($) $ in Millions		3 Months Ended			9 Months Ended	12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Reorganization Items		$ 33.9	$ 29.7	$ 95.4	$ 159.0	$ 159.0	$ 0.0	$ 0.0
Deferred Tax Assets, Valuation Allowance, Current		0.0			0.0	0.0	49.7
Deferred tax assets:
Tax credits and loss carryforwards		4,284.4			4,284.4	4,284.4	1,817.4
Accrued postretirement benefit obligations		364.5			364.5	364.5	372.4
Asset retirement obligations		163.6			163.6	163.6	160.9
Employee benefits		57.0			57.0	57.0	69.6
Payable to voluntary employee beneficiary association for certain Patriot retirees	[1]	0.0			0.0	0.0	52.9
Hedge activities		21.0			21.0	21.0	26.6
Financial guarantees		77.9			77.9	77.9	16.9
Workers' compensation obligations		7.5			7.5	7.5	13.7
Other		2.1			2.1	2.1	66.7
Total gross deferred tax assets		4,978.0			4,978.0	4,978.0	2,597.1
Deferred tax liabilities:
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments		900.4			900.4	900.4	966.6
Unamortized discount on Convertible Junior Subordinated Debentures		127.7			127.7	127.7	130.3
Investments and other assets		86.3			86.3	86.3	70.1
Total gross deferred tax liabilities		1,114.4			1,114.4	1,114.4	1,167.0
Valuation allowance, income tax		(3,881.2)			(3,881.2)	(3,881.2)	(1,447.3)
Net deferred tax liability		(17.6)			(17.6)	(17.6)	(17.2)
Deferred taxes are classified as follows:
Noncurrent deferred income taxes		(17.6)			(17.6)	(17.6)	(66.9)
Net deferred tax liability		$ (17.6)			$ (17.6)	$ (17.6)	$ (17.2)

[1] Refer to Note 27. "Matters Related to the Bankruptcy of Patriot Coal Corporation" herein for additional details related to this transaction.