Retirement Benefits - Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Sep. 30, 2012
Net periodic pension cost (credit) [Line Items]
UMWA obligation increase due to "Cadillac" plan excise tax	$ 1.3		$ 3.7	$ 7.3	$ 3.7
Retirement Benefit Plan Remeasurements [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year	15.5			15.5
Cash portion of contribution						7.0
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	$ 12.2		$ 0	$ 12.2	$ 0