Postretirement Health Care and Life Insurance Benefits (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Estimated prior service cost that will be amortized from accumulated other comprehensive (income) loss	$ (11.4)		$ 1.4		$ (20.1)
Postretirement Health Care and Life Insurance Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of actuarial gains and losses amortized	0.00%
Amortization period - future working lifetime of active employees	11.70		11.96
Amounts recognized in accumulated other comprehensive loss
Net actuarial loss (gain) arising during year	115.8		(24.3)		(68.3)
Prior service credit arising during year	(18.0)		0		(31.9)
Amortization:
Actuarial loss	(14.5)		(24.1)		(32.8)
Prior service (cost) credit	(1.3)		1.7		(2.5)
Settlement related to the Patriot bankruptcy reorganization
Actuarial loss	0	[1]	(61.3)	[1]	0	[1]
Prior service cost	0	[1]	(1.9)	[1]	0	[1]
Total recorded in other comprehensive loss (income)	82.0		(109.9)		(135.5)
Scenario, Forecast [Member] | Postretirement Health Care and Life Insurance Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Estimated net actuarial loss that will be amortized from accumulated other comprehensive (income) loss							24.9
Estimated prior service cost that will be amortized from accumulated other comprehensive (income) loss							$ 6.8

[1] Refer to Note 25. "Matters Related to the Bankruptcy Reorganization of Patriot Coal Corporation" herein for additional details related to this transaction.