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Subsequent Events (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 12 Months Ended 2 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2007
Feb. 28, 2013
Subsequent Event [Member]
Sales Agreement [Member]
Subsequent Event [Line Items]            
Common stock sold through the program (in shares) 15,333,340 5,000,000 5,185,000 869,565 3,833,108 14,200,000
Proceeds from issuance of common stock   $ 23.5 $ 33.5     $ 20.9