Line Of Credit Payable (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Revolving credit facility borrowing capacity		$ 500,000,000
Revolving credit facility weighted average outstanding balance	49,975,000
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	0		174,200,000
Line of credit facility available for future borrowings	$ 500,000,000
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%