Borrowings Under Line of Credit Arrangements and Related Items (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013 Number	Jun. 30, 2012	Jun. 30, 2013 Number	Jun. 30, 2012	Dec. 31, 2012
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 0	$ 393,000	$ 0	$ 393,000	$ 0
Maximum amount outstanding at any month end	600,000	393,000	780,000	897,000
Average amount outstanding (total of daily principal balances divided by days in period)	299,011	122,209	510,055	301,456
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.38%	1.65%	1.38%	1.65%
Borrowings Under Line of Credit Arrangements and Related Items (Textual) [Abstract]
Unsecured line of credit arrangement	2,250,000		2,250,000
Additional unsecured line of credit arrangement	1,000,000		1,000,000
Maximum aggregate commitment	3,250,000		3,250,000
Number of banks in consortium	29		29
Available to borrow in alternate currencies	$ 500,000		$ 500,000
Maximum Interest payable period in months			3
Agent bank's prime rate of interest	1.37%		1.37%
Interest Rate margin	1.175%		1.175%
Annual facility fee for each bank based on commitment amount			0.225%
Debt instrument maturity date			Mar. 31, 2017