Distributions (Details) (USD $)	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of payment of distributions to shareholders
Distributions including dividends reinvested paid to common shareholders	$ 4,316,063	$ 4,125,060	$ 3,991,110	$ 3,853,595	$ 3,656,622	$ 3,456,304	$ 3,294,938	$ 3,155,607	$ 3,062,506	$ 2,954,616	$ 2,902,806	$ 2,809,868	$ 16,285,828	$ 13,563,471	$ 11,729,796
Dividend declared per share, paid	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.72	$ 0.72	$ 0.72