Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 sqft years	Dec. 31, 2010	Dec. 31, 2009
Operating ground leases [Abstract]
Rental property expense	$ 1,400,000	$ 1,500,000	$ 1,600,000
Operating ground leases, scheduled future minimum payments [Abstract]
Operating leases, future minimum payments due, current	1,324,000
Operating leases, future minimum payments, year two	1,345,000
Operating leases, future minimum payments, year three	1,366,000
Operating leases, future minimum payments, year four	1,389,000
Operating leases, future minimum payments, year five	1,413,000
Operating leases, future minimum payments, thereafter	31,526,000
Operating leases, total future minimum payments due	38,363,000
Lease and Contractual Commitments [Abstract]
Lease and contractual commitments	59,800,000
Accrued amount of lease and contractual commitments	10,300,000
DLF I obligation [Abstract]
Excess contribution, gross	7,200,000
Term to repay excess contribution (in years)	14
Excess contribution, initial repayment date	3/12/1999
Excess contribution, net discounted amount	3,800,000
Excess contribution, discount rate	9.62%
Excess contribution, payments	600,000	600,000	600,000
Excess contribution, discounted balance	800,000	1,200,000
Non Recourse Debt [Abstract]
Secured debt of unconsolidated affiliate at disposition date		170,000,000
Rent Guarantee [Abstract]
Number of properties sold relating to rent guarantee	6
Number of customers relating to rent guarantee	1
Term of lease agreement (in years)	14
Period of rent payments in guarantee (in years)	2
Date at which rent guarantee began	2011-03
Date at which rent guarantee ends	2013-06
Square feet of lease in rent guarantee (in sq feet)	237,500
Maximum exposure under rent guarantee	$ 300,000