Retirement Benefits - Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Net periodic pension cost (credit) [Line Items]
UMWA obligation increase due to "Cadillac" plan excise tax	$ (3.8)		$ (1.0)	$ (11.1)	$ (4.7)
Retirement Benefit Plan Remeasurements [Abstract]
Cash portion of contribution	13.0			13.0
Newly issued shares of common stock for contribution		361,446
Common stock value per share for stock contribution		$ 24.90
Aggregate value of shares of common stock contribution		9.0		9.0
2010 Healthcare reform black lung obligation increase	$ (0.2)		$ 0	$ 12.0	$ 0

Before tax amount of the benefit improvement costs resulting from a plan amendment that occurred that has not been recognized in net periodic benefit cost.

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