Investments (Tables)

12 Months Ended

Dec. 31, 2015

Dec. 31, 2014

Investments, Debt and Equity Securities [Abstract]

Investments in available-for-sale securities

Investments in available-for-sale securities at December 31, 2014 were as follows:

Available-for-sale securities	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
			(Dollars in millions)
Current:
U.S. corporate bonds	11.2		—		—		11.2
Noncurrent:
Marketable equity securities	6.2		—		—		6.2
Federal government securities	32.0		—		—		32.0
U.S. corporate bonds	12.4		—		—		12.4
Total	$	61.8	$	—	$	—	$	61.8

Equity Method Investments [Table Text Block]

The table below summarizes the book value of those investments, which is reported in “Investments and other assets” in the consolidated balance sheets, and the related loss from equity affiliates:

	Book Value at December 31,				Loss from Equity Affiliates for the Year Ended December 31,
	2015		2014		2015		2014		2013
	(Dollars in millions)
Equity interest in Middlemount Coal Pty Ltd	$	—	$	58.0	$	7.0	$	98.5	$	33.5
Other equity method investments	4.7		7.3		8.9		9.1		6.7
Total equity method investments	$	4.7	$	65.3	$	15.9	$	107.6	$	40.2