Subsequent Events (Details) (USD $)	3 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Subsequent Events (Details)
Distribution declaration date	Oct. 18, 2012	Jul. 19, 2012	Apr. 20, 2012	Jan. 20, 2012
Amount of declared distribution	$ 0.3975	$ 0.3875	$ 0.3875	$ 0.3875
Amount of declared distribution on an annualized basis	$ 1.59	$ 1.55	$ 1.55	$ 1.55
Distribution payment date	Nov. 15, 2012	Aug. 15, 2012	May 15, 2012	Feb. 15, 2012
Distribution record date	Nov. 01, 2012	Aug. 01, 2012	May 01, 2012	Feb. 01, 2012