Line Of Credit Payable (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Oct. 31, 2012
Line of Credit Facility [Line Items]
Revolving credit facility borrowing capacity			$ 500,000,000
Revolving credit facility weighted average outstanding balance	127,198,000
Revolving credit facility weighted average interest rate	1.40%
Maturity date	Oct. 01, 2016
Option to extend maturity date	Oct. 01, 2017
Option to increase facility size	1,000,000,000
Line of credit payable	59,400,000	174,200,000
Line of credit facility available for future borrowings	$ 440,600,000
Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt instrument, description of variable rate basis	LIBOR plus 117.5 basis points
Basis spread on variable rate	1.175%