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Public Equity Offerings - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
0 Months Ended
Sep. 19, 2012
Nov. 15, 2011
Nov. 10, 2010
Dec. 31, 2012
Oct. 03, 2012
Dec. 31, 2011
Nov. 16, 2011
Nov. 11, 2010
Equity [Line Items]                
Common stock, issued 5,000,000 4,000,000 5,750,000 38,829,368   38,829,368    
Additional purchase of common stock         325,000   600,000 862,500
Transaction costs $ 8.0 $ 6.6 $ 5.7          
Received net proceeds $ 154.4 $ 122.2 $ 101.8