Borrowings Under Line of Credit Arrangement and Related Items (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jul. 27, 2011	Jan. 24, 2011	Dec. 31, 2010
Aggregate borrowings under the unsecured line of credit arrangement
Balance outstanding at quarter end	$ 0	$ 206,000,000	$ 0	$ 206,000,000			$ 300,000,000	[1]
Maximum amount outstanding at any month end	20,000,000	431,000,000	495,000,000	431,000,000
Average amount outstanding (total of daily principal balances divided by days in period)	253,000	293,505,000	158,856,000	288,337,000
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)		1.63%	2.17%	1.55%
Borrowings Under Line of Credit Arrangement and Related Items (Textuals) [Abstract]
Unsecured line of credit arrangement	1,150,000,000		1,150,000,000
Number of banks in consortium	16		16
Maximum Interest payable period in months			P3M
Libor Rate	0.79%		0.79%
Applicable margin Over LIBOR, based on certain debt ratings	0.60%		0.60%		1.35%
Annual facility fee for each bank based on commitment amount	0.15%		0.15%		0.25%
Annual agent's fee	50,000		50,000
Line of credit extension fee						$ 1,725,000

[1] NOTE: The consolidated balance sheet at December 31, 2010 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by U.S. generally accepted accounting principles for complete financial statements.