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Organization and Capitalization - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Segment
Dec. 31, 2013
Dec. 06, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2009
Dec. 31, 2007
Dec. 27, 2007
Jul. 12, 2006
Organization and Capitalization [Line Items]                  
Common stock, shares authorized 750,000,000us-gaap_CommonStockSharesAuthorized 750,000,000us-gaap_CommonStockSharesAuthorized   130,322,900us-gaap_CommonStockSharesAuthorized 126,863,000us-gaap_CommonStockSharesAuthorized 38,635,550us-gaap_CommonStockSharesAuthorized 14,416,250us-gaap_CommonStockSharesAuthorized   11,533,000us-gaap_CommonStockSharesAuthorized
Preferred stock, shares authorized     25,000,000us-gaap_PreferredStockSharesAuthorized 65,693,245us-gaap_PreferredStockSharesAuthorized 62,693,245us-gaap_PreferredStockSharesAuthorized 21,734,402us-gaap_PreferredStockSharesAuthorized   6,500,000us-gaap_PreferredStockSharesAuthorized  
Preferred stock, per share     $ 0.01us-gaap_PreferredStockParOrStatedValuePerShare            
Number of business segments 1us-gaap_NumberOfReportableSegments                
Minimum [Member]                  
Organization and Capitalization [Line Items]                  
Common stock, shares authorized     225,000,000us-gaap_CommonStockSharesAuthorized
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		            
Maximum [Member]                  
Organization and Capitalization [Line Items]                  
Common stock, shares authorized     750,000,000us-gaap_CommonStockSharesAuthorized
/ us-gaap_RangeAxis
= us-gaap_MaximumMember