Borrowings Under Line of Credit Arrangements and Related Items (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Number	Mar. 31, 2011	Dec. 31, 2011
Aggregate borrowings under the unsecured line of credit arrangements
Balance outstanding at quarter end	$ 5,000	$ 0	$ 610,000
Maximum amount outstanding at any month end	897,000	495,000
Average amount outstanding (total of daily principal balances divided by days in period)	480,703	319,222
Weighted average interest rate (actual interest expense divided by average borrowings outstanding)	1.65%	1.59%
Borrowings Under Line of Credit Arrangements and Related Items (Textual) [Abstract]
Unsecured line of credit arrangement	2,000,000
Additional unsecured line of credit arrangement	500,000
Maximum aggregate commitment	$ 2,500,000
Number of banks in consortium	31
Maximum Interest payable period in months	3
Agent bank's prime rate of interest	1.60%
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%