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Asset Retirement Obligations - Change in Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]    
Balance at beginning of year $ 13.0 $ 12.8
Accretion expense 1.1 1.1
Payments (0.8) (0.6)
Revision of estimates 13.8 (0.3)
Liabilities incurred 0.6 0
Balance at end of year $ 27.7 $ 13.0