Unsecured Debt (Details 1) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Summary of short-term bank borrowings under unsecured commercial bank credit facility
Total revolving credit facility,	$ 1,400,000
Unsecured Commercial Bank Credit Facility [Member]
Summary of short-term bank borrowings under unsecured commercial bank credit facility
Total revolving credit facility,	600,000	600,000
Borrowings outstanding at end of period	5,000	31,750
Weighted average borrowings during the period ended	125,719	161,260
Maximum daily borrowings during the period ended	$ 335,300	$ 337,600
Weighted average interest rate during the period ended	0.70%	0.80%
Interest rate at the end of the period	0.80%	0.90%