Line Of Credit Payable (Details) (USD $)	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	May 31, 2011
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	59,015,000
Revolving credit facility weighted average interest rate	1.80%
Maturity date	May 31, 2015
Option to extend maturity date	May 2016.
Debt instrument, description of variable rate basis	LIBOR plus 150
Basis spread on variable rate	1.50%
Option to increase facility size	650,000,000
Line of credit payable	0	65,600,000
Line of credit facility available for future borrowings	450,000,000
Line of credit facility undrawn letters of credit	$ 3,800,000