Stockholders' Equity (Details) (USD $)	3 Months Ended		6 Months Ended		1 Months Ended				1 Months Ended					2 Months Ended
	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Series A Preferred Stock	Apr. 30, 2012 Series A Preferred Stock	Nov. 30, 2011 Series A Preferred Stock	Nov. 02, 2011 Series A Preferred Stock	Jul. 31, 2012 Common Shares	May 31, 2012 Common Shares	Apr. 30, 2012 Common Shares	May 31, 2011 Common Shares	Apr. 30, 2011 Common Shares	May 31, 2011 Common Shares	May 29, 2012 Common Shares	Apr. 20, 2011 Common Shares
Stockholders' equity
Authorized shares of preferred stock		10,000,000	10,000,000	10,000,000
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01	$ 0.01
Number of shares sold							2,760,000			8,337,500			13,750,000
Number of shares issued pursuant to the full exercise of underwriters' overallotment option							360,000			1,087,500		2,062,500
Dividend rate (as a percent)							9.00%
Issue price (in dollars per share)								$ 25.00				$ 13		$ 13	$ 12.88	$ 13
Net proceeds from sale of shares							$ 66,300,000
Direct offering costs	17,042,000		5,104,000				2,700,000					16,900,000	12,500,000
Indirect offering costs		68,000	68,000				100,000			100,000
Dividend paid						1,600,000
Dividend declared					1,600,000
Common stock, outstanding (in shares)		24,958,258	24,958,258	15,901,560
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01	$ 0.01
Net proceeds from the offering										102,800,000		24,900,000	166,300,000
Gross proceeds from the offering	205,563,000		107,387,000							107,400,000		26,800,000	178,800,000	205,600,000
Underwriting fees incurred reflected as a reduction to additional paid-in capital										4,600,000
Direct offering costs incurred reflected as a reduction to additional paid-in capital										500,000
Formation transaction costs and offering costs incurred														6,200,000
Portion of formation transaction costs and offering costs expensed	3,728,000
Portion of formation transaction costs and offering costs deducted from the gross proceeds of the IPO														2,500,000
Dividend paid (in dollars per share)											$ 0.26
Dividend declared (in dollars per share)	$ 0.2057	$ 0.27	$ 0.53							$ 0.27
Issuance of shares to directors for their services									3,108		3,776
Fair value of shares issued to directors for their services									$ 46,000		$ 50,000