Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Future minimum lease and royalty payments
Capital Leases, 2016	$ 12.9
Capital Leases, 2017	7.3
Capital Leases, 2018	8.8
Capital Leases, 2019	0.5
Capital Leases, 2020	0.5
Capital Leases, 2021 and Thereafter	10.1
Capital Leases, Total minimum lease payments	40.1
Capital Leases, Less interest	9.8
Capital Leases, Present value of minimum capital lease payments	30.3
Operating Leases, 2016	191.5
Operating Leases, 2017	173.8
Operating Leases, 2018	109.4
Operating Leases, 2019	64.2
Operating Leases, 2020	23.4
Operating Leases, 2021 and thereafter	36.5
Operating Leases, Total minimum lease payments	598.8
Coal Lease and Royalty Obligation, 2016	254.3
Coal Lease and Royalty Obligation, 2017	20.3
Coal Lease and Royalty Obligation, 2018	19.9
Coal Lease and Royalty Obligation, 2019	19.4
Coal Lease and Royalty Obligation, 2020	18.9
Coal Lease and Royalty Obligation, 2021 and Thereafter	30.8
Coal Lease and Royalty Obligation, Total minimum lease payments	363.6
Leases (Textuals)
Rental expense under operating leases	290.1	$ 306.0	$ 305.9
Contingent lease expense	0.0	0.0	0.0
Property, plant, equipment and mine development assets, gross value under capital leases	125.6	175.1
Accumulated depreciation of property, plant, equipment and mine development assets under capital leases	111.4	138.4
Total royalty expenses on coal reserve leases	$ 444.5	$ 507.8	$ 546.0
Initial lease term for federal leases	ten years
Minimum annual production on federal leases	1.00%
Monthly royalty percentage on federal leases for coal sales using surface mining methods	12.50%
Monthly royalty percentage on federal leases for coal production using underground mining methods	8.00%
Period to redetermine royalty rates on leased coal reserves in Arizona	every ten years
Company's lease obligations secured by outstanding surety bonds	$ 110.5