Investments (Details 1) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2014	Jun. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 61.8			$ 61.8
Available-for-sale Securities, Gross Unrealized Gain				0.0
Available-for-sale Securities, Gross Unrealized Loss				0.0
Available-for-sale Securities	61.8			61.8
Proceeds from sale of available-for-sale securities			$ 90.3	13.5	$ 22.9
Net gains on sales and maturities			0.0	0.0	0.2
Proceeds from Sale and Maturity of Marketable Securities			90.3	13.5	22.9
Other-than-temporary impairment losses on marketable securities			0.0	4.7	21.5
Payments to Acquire Short-term Investments						$ 4.8
Maturity of short-term investments			$ 0.0	0.0	$ 4.8
Held-to-maturity Securities	0.0			0.0
Current [Member] | Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	11.2			11.2
Available-for-sale Securities, Gross Unrealized Gain				0.0
Available-for-sale Securities, Gross Unrealized Loss				0.0
Available-for-sale Securities	11.2			11.2
Non Current [Member] | Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	12.4			12.4
Available-for-sale Securities, Gross Unrealized Gain				0.0
Available-for-sale Securities, Gross Unrealized Loss				0.0
Available-for-sale Securities	12.4			12.4
Non Current [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	6.2			6.2
Available-for-sale Securities, Gross Unrealized Gain				0.0
Available-for-sale Securities, Gross Unrealized Loss				0.0
Available-for-sale Securities	6.2			6.2
Non Current [Member] | US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	32.0			32.0
Available-for-sale Securities, Gross Unrealized Gain				0.0
Available-for-sale Securities, Gross Unrealized Loss				0.0
Available-for-sale Securities	32.0			32.0
Interest In Peabody-Winsway Resources B.V. [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairment losses on marketable securities	4.7	$ 21.5
Middlemount Mine [Member]
Schedule of Available-for-sale Securities [Line Items]
Valuation Allowances and Reserves, Balance	$ 52.3			$ 52.3