Borrowings Under Line of Credit Arrangements and Related Items (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012 Number	Sep. 30, 2011	Sep. 30, 2012 Number	Sep. 30, 2011	Dec. 31, 2011
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 0	$ 390,000	$ 0	$ 390,000	$ 610,000
Maximum amount outstanding at any month end	145,000	390,000	897,000	495,000
Average amount outstanding (total of daily principal balances divided by days in period)	165,000	140,978	255,639	152,832
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	2.30%	1.61%	1.79%	1.12%
Borrowings Under Line of Credit Arrangements and Related Items (Textual) [Abstract]
Unsecured line of credit arrangement	2,000,000		2,000,000
Additional unsecured line of credit arrangement	500,000		500,000
Maximum aggregate commitment	2,500,000		2,500,000
Number of banks in consortium	31		31
Maximum Interest payable period in months	3
Agent bank's prime rate of interest	1.57%		1.57%
Applicable margin over LIBOR, based on certain debt ratings	1.35%
Annual facility fee for each bank based on commitment amount	0.25%
Unsecured revolving demand note margin over LIBOR	1.10%
Unsecured Revolving Demand Note	$ 5,000		$ 5,000