Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Total asset retirement obligation	$ 712.1	$ 752.5	$ 712.8
Current portion	25.5	30.2
Liabilities incurred or acquired	1.3	22.7
Liabilities settled or disposed	(53.3)	(19.7)
Accretion expense	42.7	39.3
Revision to estimates	(31.1)	(2.6)
Noncurrent obligation	686.6	722.3
Balance at end of year — active locations	656.8	676.2
Balance at end of year — closed or inactive locations	55.3	76.3
Gain on disposal of asset	$ 45.0	$ 41.4	$ 52.6
Credit adjusted, risk-free interest rates	50.83%	6.82%	6.44%
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities	$ 609.4	$ 645.0
Amount of reclamation self-bonding in certain states in which the company qualifies	1,430.8	1,361.4
Letters of credit in support of reclamation obligations or activities	126.6	$ 17.6
Eastern US Coal Reserves [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Liabilities incurred or acquired	22.2
Interest in Coal Reserve Property [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Gain on disposal of asset	$ 9.6