Line Of Credit Payable (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	May 31, 2011	May 30, 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	50,767,000
Revolving credit facility weighted average interest rate	1.80%
Maturity date	May 2015
Option to extend maturity date	May 2016.
Spread over LIBOR	LIBOR plus 150
Option to increase facility size	650,000,000
Line of credit payable	26,200,000	65,600,000
Line of credit facility available for future borrowings	423,800,000
Line of credit facility undrawn letters of credit	$ 3,857,000