Investments (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 61.8	$ 65.7		$ 61.8
Available-for-sale Securities, Gross Unrealized Gain	0	1.4
Available-for-sale Securities, Gross Unrealized Loss	0	(0.1)
Available-for-sale Securities	61.8	67.0		61.8
Proceeds from sale of available-for-sale securities	13.5	22.9	17.2
Net gains on sales and maturities	0	0.2	0.1
Proceeds from Sale and Maturity of Marketable Securities	13.5	22.9	46.4
Other-than-temporary impairment losses on marketable securities	4.7	21.5	35.5
Purchases of short-term investments	0	0	4.8
Maturity of short-term investments	0	4.8	0
Held-to-maturity securities	0	0		0
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, at cost due within one year	11.2			11.2
Available-for-sale securities, at fair value due within one year	11.2			11.2
Available-for-sale securities, at cost due in one to five years	44.4			44.4
Available-for-sale securities, at fair value due in one to five years	44.4			44.4
Available-for-sale securities, at cost	55.6			55.6
Available-for-sale securities, at fair value	55.6			55.6
Current [Member] | Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	11.2	2.4		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	2.4		11.2
Current [Member] | US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis		2.8
Available-for-sale Securities, Gross Unrealized Gain		0
Available-for-sale Securities, Gross Unrealized Loss		0
Available-for-sale Securities		2.8
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	20.8		12.4
Available-for-sale Securities, Gross Unrealized Gain	0	0.1
Available-for-sale Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities	12.4	20.9		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	10.9		6.2
Available-for-sale Securities, Gross Unrealized Gain	0	1.3
Available-for-sale Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities	6.2	12.2		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	28.8		32.0
Available-for-sale Securities, Gross Unrealized Gain	0	0
Available-for-sale Securities, Gross Unrealized Loss	0	(0.1)
Available-for-sale Securities	32.0	28.7		32.0
Newcastle Coal Infrastructure Group [Domain]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sale and Maturity of Marketable Securities			29.2
Realized losses on sale securities			0.2
NCIG Securities	0	0		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	$ 35.5