Transactions with Related Parties - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012 Person Y	Dec. 31, 2011	Dec. 31, 2010
Related Party Transaction [Line Items]
Number of independent directors									4
Management agreement initial expiration date									February 23, 2015
Management agreement additional term of expiration (in years)									1
Prior notice for uncertain termination (in days)									180 days
Proportion of average annual management fee									4
Period of annual management fees earned (in years)									2
Management fee payable	$ 2,120,000				$ 1,200,000				$ 2,120,000	$ 1,200,000
Share based compensation expense	619,000	447,000	425,000	429,000	415,000	334,000	194,000	207,000	1,920,000	1,150,000	1,432,000
Company's Equity Up To $250 Million
Related Party Transaction [Line Items]
Management fee bases per annum									1.50%
Company's Equity Up To $250 Million | Maximum
Related Party Transaction [Line Items]
Management fee payable									250,000,000
Company's Equity In Excess Of $250 Million And Up To $500 Million
Related Party Transaction [Line Items]
Management fee bases per annum									1.10%
Company's Equity In Excess Of $250 Million And Up To $500 Million | Maximum
Related Party Transaction [Line Items]
Management fee payable									500,000,000
Company's Equity In Excess Of $250 Million And Up To $500 Million | Minimum
Related Party Transaction [Line Items]
Management fee payable									250,000,000
Company's Equity In Excess Of $500 Million And Up To $750 Million
Related Party Transaction [Line Items]
Management fee bases per annum									0.80%
Company's Equity In Excess Of $500 Million And Up To $750 Million | Maximum
Related Party Transaction [Line Items]
Management fee payable									750,000,000
Company's Equity In Excess Of $500 Million And Up To $750 Million | Minimum
Related Party Transaction [Line Items]
Management fee payable									500,000,000
Company's Equity In Excess Of $750
Related Party Transaction [Line Items]
Management fee bases per annum									0.50%
Company's Equity In Excess Of $750 | Minimum
Related Party Transaction [Line Items]
Management fee payable									$ 750,000,000