Subsequent Events (Details) (USD $) In Thousands, except Share data, unless otherwise specified			0 Months Ended			9 Months Ended		1 Months Ended	9 Months Ended			9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Oct. 05, 2012 Class B-1 common stock	Sep. 30, 2012 Mortgages payable	Dec. 31, 2011 Mortgages payable	Sep. 30, 2012 Subsequent events Class B-1 common stock	Sep. 30, 2012 Subsequent events Bakersfield, California sqft	Sep. 30, 2012 Subsequent events Columbus, Ohio sqft	Sep. 30, 2012 Subsequent events Columbus, Ohio sqft	Sep. 30, 2012 Subsequent events El Paso, Texas sqft	Sep. 30, 2012 Subsequent events Mortgages payable property	Sep. 30, 2012 Subsequent events Senior unsecured revolving line of credit
Subsequent Events
Amount drawn												$ 65,000
Amount outstanding	2,396,899	2,926,218		2,271,899	2,779,777						77,392
Number of properties pledged as collateral											5
Weighted-average interest rate (as a percent)	5.77%			6.11%	6.13%						5.72%
Square footage							75,100	116,100	116,100	75,500
Sales price							3,250		22,400	7,750
Gain								$ 5,457
Shares of Class B common stock converted to Class A common stock			48,518			48,518,000