Other Assets (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Other Assets
Deferred leasing costs	$ 14,886	$ 11,601
Deferred rent receivable	15,017	11,051
Deferred financing costs	2,520	3,607
Leasehold interests in corporate headquarters	4,152	2,719
Other	7,981	4,765
Total	44,556	33,743
Estimated amortization of deferred leasing costs
2013	3,396
2014	3,043
2015	2,056
2016	1,687
2017	1,161
Thereafter	3,543
Total	$ 14,886	$ 11,601