Postretirement Health Care and Life Insurance Benefits (Details 1) $ in Millions		12 Months Ended
		Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) years	Dec. 31, 2014 USD ($) years	Dec. 31, 2013 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Estimated prior service cost that will be amortized from accumulated other comprehensive (income) loss			$ (10.4)	$ (11.4)	$ 1.4
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 | years			10.49	11.00
Amounts recognized in accumulated other comprehensive loss
Net actuarial loss (gain) arising during year			$ (35.1)	$ 115.8	(24.3)
Prior service credit arising during year			0.0	(18.0)	0.0
Amortization:
Actuarial loss			(24.9)	(14.5)	(24.1)
Prior service credit (cost)			6.8	(1.3)	1.7
Settlement related to the Patriot bankruptcy
Actuarial loss	[1]		0.0	0.0	(61.3)
Prior service cost	[1]		(16.6)	0.0	(1.9)
Total recorded in other comprehensive (income) loss			$ (69.8)	$ 82.0	$ (109.9)
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		$ 20.4
Estimated prior service cost that will be amortized from accumulated other comprehensive (income) loss		$ 11.0

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