Borrowings Under Line of Credit Arrangement and Related Items (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Jul. 27, 2011 Bank	Dec. 31, 2010
Aggregate borrowings under the unsecured line of credit arrangement
Balance outstanding at quarter end	$ 390,000,000	$ 0	$ 390,000,000	$ 0		$ 300,000,000	[1]
Maximum amount outstanding at any month end	390,000,000	560,000,000	495,000,000	560,000,000
Average amount outstanding (total of daily principal balances divided by days in period)	140,978,000	220,467,000	152,832,000	265,465,000
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	4.58%	2.22%	2.91%	1.74%
Borrowings Under Line of Credit Arrangement and Related Items (Textual) [Abstract]
Unsecured line of credit arrangement					2,000,000,000
Additional unsecured line of credit arrangement					500,000,000
Maximum aggregate commitment					$ 2,500,000,000
Number of banks in consortium					31
Maximum Interest payable period in months			P3M
Agent bank's prime rate of interest	1.59%		1.59%
Applicable margin Over LIBOR, based on certain debt ratings	1.35%		1.35%
Annual facility fee for each bank based on commitment amount	0.25%		0.25%

[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.