Organization and basis of presentation (Details) (USD $)	0 Months Ended	3 Months Ended		4 Months Ended	6 Months Ended	9 Months Ended
	May 01, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2013	Sep. 30, 2013	Sep. 30, 2013 Secured Debt [Member]	Sep. 30, 2013 Secured Debt [Member] Repurchase agreement due March 21, 2014	Mar. 22, 2013 Secured Debt [Member] Repurchase agreement due March 21, 2014	Sep. 30, 2013 Secured Debt [Member] Repurchase agreement due March 11, 2016	Sep. 12, 2013 Secured Debt [Member] Repurchase agreement due March 11, 2016	Sep. 30, 2013 Secured Debt [Member] Repurchase agreement due March 23, 2015	Sep. 23, 2013 Secured Debt [Member] Repurchase agreement due March 23, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Proceeds from contributed capital			$ 100,000,000
Ownership percentage in general partner of the operating partnership		100.00%				100.00%
Ownership interest in operating partnerhip		100.00%				100.00%
Minimum distribution percentage of REIT taxable income		90.00%			90.00%
Maximum borrowing capacity							425,000,000	100,000,000	100,000,000	125,000,000	125,000,000	200,000,000	200,000,000
Sale of Stock, Number of Shares Issued in Transaction	17,250,000
Sale of Stock, Price Per Share	$ 18.75
Issuance of common stock, including stock option exercises	$ 309,500,000			$ 500,000		$ 323,619,000