Line Of Credit Payable (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Line of Credit Facility [Abstract]
Revolving credit facility original borrowing capacity	$ 400,000,000
Revolving credit facility borrowing capacity	450,000,000
Revolving credit facility weighted average outstanding balance	104,644,000
Revolving credit facility weighted average interest rate	3.20%
Maturity date	May 2015
Option to extend maturity date	May 2016.
Option to increase facility size	650,000,000
Line of credit payable	65,600,000	161,000,000
Line of credit facility available for future borrowings	384,400,000
Line of credit facility undrawn letter of credit	$ 57,000