Secured and Unsecured Debt (Details 4) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Summary of short-term bank borrowings under unsecured commercial bank credit facility
Borrowings outstanding at end of period	$ 839,700
Unsecured Commercial Bank Credit Facility [Member]
Summary of short-term bank borrowings under unsecured commercial bank credit facility
Total revolving credit facility	900,000		900,000
Borrowings outstanding at end of period	228,500		76,000
Weighted average daily borrowings during the period ended	179,495	167,038
Maximum daily borrowings during the period ended	$ 372,000	$ 788,000
Weighted average interest rate during the period ended	1.20%	1.50%
Interest rate at the end of the period	1.30%		1.40%