Borrowings Under Line of Credit Arrangements and Related Items (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Number	Mar. 31, 2012	Dec. 31, 2012
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 710,000	$ 5,000	$ 0
Maximum amount outstanding at any month end	780,000	897,000
Average amount outstanding (total of daily principal balances divided by days in period)	723,444	480,703
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.38%	1.65%
Borrowings Under Line of Credit Arrangements and Related Items (Textual) [Abstract]
Unsecured line of credit arrangement	2,250,000
Additional unsecured line of credit arrangement	1,000,000
Maximum aggregate commitment	3,250,000
Number of banks in consortium	29
Available to borrow in alternate currencies	500,000
Maximum Interest payable period in months	3
Agent bank's prime rate of interest	1.38%
Interest Rate margin	1.175%
Annual facility fee for each bank based on commitment amount	0.225%
Unsecured Revolving Demand Note	$ 10,000
Unsecured Revolving Demand Note Rate	1.13%
Unsecured revolving demand note margin over LIBOR	1.10%
Debt instrument maturity date	Mar. 31, 2017