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Commitments and Contingencies - Public offering and controlled equity offering (Details) - Public Offering And Controlled Equity Offering - USD ($)
Jan. 25, 2013
May 27, 2016
Maximum [Member]    
Other Commitments [Line Items]    
Aggregate Initial Offering Price   $ 250,000,000
Selling Agents [Member]    
Other Commitments [Line Items]    
Commission as Percentage of Gross Proceeds 3.00%  
Selling Agents [Member] | Maximum [Member]    
Other Commitments [Line Items]    
Aggregate Initial Offering Price $ 30,000,000