Condensed Statements Of Income (Parenthetical) (USD $)	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2013	Mar. 31, 2012	Jun. 30, 2013
Distributions declared per share	$ 2.50		$ 3.00		$ 2.25		$ 2.25
Dividends declared date		Jun. 17, 2013		Jun. 06, 2012		Mar. 08, 2013		Mar. 09, 2012
Dividends payable date		Jul. 31, 2013		Jul. 31, 2012		Apr. 30, 2013		Apr. 30, 2012