Credit Facility (Details) (USD $)	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Line of Credit Facility [Line Items]
Line of Credit Outstanding Balance	$ 96,600,000		$ 0
Revolving Unsecured Credit Facility [Member]
Line of Credit Facility [Line Items]
Revolving unsecured credit facility	425,000,000
Initiation date	December 2010
Expiration date	March 2014
Available one-year extension options	2
Line of credit facility, interest rate description	London Interbank Offered Rate, commonly referred to as LIBOR, plus 185 basis points with a facility commitment fee of 35 basis points, for all-in drawn pricing of 220 basis points over LIBOR. The borrowing rate is not subject to an interest rate floor or ceiling
Variable interest rate, LIBOR plus basis points	185
Line of credit facility, commitment fee basis points	35
Line of credit facility, all-in drawn variable interest rate	220
Unamortized debt issuance expense	3,500,000		4,200,000
Line of Credit Borrowing Capacity	328,400,000
Line of Credit Outstanding Balance	96,600,000
Average borrowing rate during period	2.10%	1.30%
Revolving Unsecured Credit Facility Replaced [Member]
Line of Credit Facility [Line Items]
Revolving unsecured credit facility	$ 355,000,000
Expiration date	May 2011