Credit Facility (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended			9 Months Ended		9 Months Ended								9 Months Ended
	Sep. 30, 2012 financial_institutions	Jul. 31, 2012	Dec. 31, 2011	Sep. 30, 2012 Amended and restated credit facility rating_agency	Feb. 24, 2012 Amended and restated credit facility	Sep. 30, 2012 Amended and restated credit facility Investment grade rated	Sep. 30, 2012 Amended and restated credit facility Minimum	Sep. 30, 2012 Amended and restated credit facility Minimum Investment grade rated	Sep. 30, 2012 Amended and restated credit facility Maximum	Sep. 30, 2012 Amended and restated credit facility Maximum Investment grade rated	Feb. 24, 2012 Senior unsecured revolving line of credit	Sep. 30, 2012 Unsecured term loan	Feb. 24, 2012 Unsecured term loan	Sep. 30, 2012 Prior secured credit facility	Dec. 31, 2011 Prior secured credit facility	Sep. 30, 2012 Prior secured credit facility Minimum	Sep. 30, 2012 Prior secured credit facility Maximum	Sep. 30, 2012 Prior senior secured revolving line of credit	Sep. 30, 2012 Prior secured term loan
Credit Facility
Amount borrowed					$ 650,000						$ 350,000			$ 585,000				$ 435,000
Principal amount													300,000						150,000
Maximum borrowing capacity					850,000
Period of extension of maturity (in years)				1 year
Extension fee as a percentage of commitment amount				0.25%
Reference rate for variable interest rate				LIBOR		LIBOR								LIBOR
Variable interest rate spread (as a percent)							1.75%	1.15%	2.50%	1.95%						2.75%	4.00%
Number of financial institutions associated with Company's unsecured credit facility that is counterparty to the July 2012 interest rate swap	1
Variable rate debt swapped to fixed rate debt	76,109	300,000	76,269
Derivative reference rate	one-month floating rate LIBOR
Fixed interest rate (as a percent)	0.53875%	0.53875%
Number of major rating agencies required to rate Company				2
Number of major rating agencies				3
Unused fees (as a percent)							0.25%	0.20%	0.35%	0.45%
Maximum advance rate on the implied value of the unencumbered pool assets (as a percent)				60.00%
Capitalization rate applied to adjusted operating income for determining maximum advance rate (as a percent)				7.50%
Amount of recourse cross-default permissions				20,000
Amount of non-recourse cross-default permissions				100,000
Non-recourse indebtedness in default				26,865
Period of permissions stipulated for non-recourse debt default (in years)									90 days
Interest rate (as percent)				2.50%								2.79%
Outstanding balance				235,000											555,000
Amount of borrowing capacity currently available				$ 115,000