Postretirement Health Care and Life Insurance Benefits (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of net periodic postretirement benefit cost
Settlement related to the Patriot bankruptcy reorganization		$ 0.0	$ 0.0	$ 30.6
Postretirement Health Care and Life Insurance Benefits [Member]
Components of net periodic postretirement benefit cost
Service cost for benefits earned		11.2	12.2	15.8
Interest cost on accumulated postretirement benefit obligation		33.8	36.4	41.8
Amortization of prior service (credit) cost		(6.8)	1.3	(1.7)
Amortization of actuarial loss		24.9	14.5	24.1
Settlement related to the Patriot bankruptcy reorganization	[1]	0.0	0.0	63.2
Special termination benefits	[2]	0.0	1.6	0.9
Total net periodic postretirement or pension cost		$ 63.1	$ 66.0	$ 144.1

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

[2] Reflected in "Restructuring and pension settlement charges" in the consolidated statement of operations for the year ended December 31, 2014.