Condensed Statements Of Income (Parenthetical) (USD $)	0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended
	Sep. 13, 2013 Dividends Declared, 9/13/2013 [Member]	Sep. 30, 2013 Dividends Declared, 9/13/2013 [Member]	Sep. 07, 2012 Dividends Declared, 9/7/2012 [Member]	Sep. 30, 2013 Dividends Declared, 9/7/2012 [Member]	Mar. 08, 2013 Dividends Declared, 3/8/2013 [Member]	Sep. 30, 2013 Dividends Declared, 3/8/2013 [Member]	Jun. 17, 2013 Dividends Declared, 6/17/2013 [Member]	Sep. 30, 2013 Dividends Declared, 6/17/2013 [Member]	Sep. 13, 2013 Dividends Declared, 9/13/201 [Member]	Sep. 30, 2013 Dividends Declared, 9/13/201 [Member]	Mar. 09, 2012 Dividends Declared, 3/9/2012 [Member]	Sep. 30, 2013 Dividends Declared, 3/9/2012 [Member]	Jun. 06, 2012 Dividends Declared, 6/6/2012 [Member]	Sep. 30, 2013 Dividends Declared, 6/6/2012 [Member]	Sep. 07, 2012 Dividends Declared, 9/7/2012 [Member]	Sep. 30, 2013 Dividends Declared, 9/7/2012 [Member]
Distributions declared	$ 2.60		$ 3.50		$ 2.25		$ 2.50		$ 2.60		$ 2.25		$ 3.00		$ 3.50
Distributions date declared		Sep. 13, 2013		Sep. 07, 2012		Mar. 08, 2013		Jun. 17, 2013		Sep. 13, 2013		Mar. 09, 2012		Jun. 06, 2012		Sep. 07, 2012
Distributions date payable		Oct. 31, 2013								Oct. 31, 2013
Distributions date paid				Oct. 31, 2012		Apr. 30, 2013		Jul. 31, 2013				Apr. 30, 2012		Jul. 31, 2012		Oct. 31, 2012