Condensed Statements Of Income (Parenthetical) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Distributions declared per share	$ 3.50	[1]	$ 4.00	[2]	$ 8.75	[3]	$ 9.25	[4]
Three Months Ended Quarter 3, 2012 [Member]
Distributions declared per share	$ 3.50
Dividends declared date					Sep. 07, 2012
Dividends payable date					Oct. 31, 2012
Three Months Ended Quarter 3, 2011 [Member]
Distributions declared per share			$ 4.00
Dividends declared date					Sep. 16, 2011
Dividends payable date					Oct. 31, 2011
Nine Months Ended Quarter 3, 2012, Dividend 1 [Member]
Distributions declared per share					$ 2.25
Dividends declared date					Mar. 09, 2012
Dividends payable date					Apr. 30, 2012
Nine Months Ended Quarter 3, 2012, Dividend 2 [Member]
Distributions declared per share					$ 3.00
Dividends declared date					Jun. 06, 2012
Dividends payable date					Jul. 31, 2012
Nine Months Ended Quarter 3, 2012, Dividend 3 [Member]
Distributions declared per share					$ 3.50
Dividends declared date					Sep. 07, 2012
Dividends payable date					Oct. 31, 2012
Nine Months Ended Quarter 3, 2011, Dividend 1 [Member]
Distributions declared per share							$ 2.25
Dividends declared date					Mar. 10, 2011
Dividends payable date					Apr. 29, 2011
Nine Months Ended Quarter 3, 2011, Dividend 2 [Member]
Distributions declared per share							$ 3.00
Dividends declared date					Jun. 10, 2011
Dividends payable date					Jul. 29, 2011
Nine Months Ended Quarter 3, 2011, Dividend 3 [Member]
Distributions declared per share							$ 4.00
Dividends declared date					Sep. 16, 2011
Dividends payable date					Oct. 31, 2011

[1] $3.50 declared 9/7/2012 payable 10/31/2012

[2] $4.00 declared 9/16/2011 paid 10/31/2011

[3] $2.25 declared 3/9/2012 paid 4/30/2012 plus $3.00 declared 6/6/2012 paid 7/31/2012 plus $3.50 declared 9/7/2012 payable 10/31/2012

[4] $2.25 declared 3/10/2011 paid 4/29/2011 plus $3.00 declared 6/10/2011 paid 7/29/2011 plus $4.00 declared 9/16/2011 paid 10/31/2011