Credit Facility (Details) (USD $)	1 Months Ended	9 Months Ended
	May 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Apr. 30, 2012	Dec. 31, 2011
Credit facility
Origination costs incurred		$ 7,069,000	9,923,000
Remaining origination costs from replaced credit facility		2,400,000
Outstanding balance		609,000,000			237,400,000
Unsecured revolving credit facility
Credit facility
Maximum borrowing capacity	1,000,000,000			425,000,000
Term of extension option		1 year
Line of credit facility, variable reference rate		LIBOR
Variable interest rate, basis points spread over variable reference rate (as a percent)		1.075%
Line of credit facility, commitment fee basis points (as a percent)		0.175%
Line of credit facility, all-in drawn variable interest rate (as a percent)		1.25%
Origination costs incurred	7,100,000	6,400,000
Current borrowing capacity available		391,000,000
Outstanding balance		$ 609,000,000			$ 237,400,000
Average borrowing rate during the period (as a percent)		1.60%	2.10%
Effective interest rate (as a percent)		1.30%