Real Estate Properties (Details 4) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 sqft	Dec. 31, 2012	Dec. 31, 2011
Real Estate Properties
Expenditures committed on leases executed during 2012	$ 128,486
Leases executed in 2013, square feet	5,152,000
Committed but unspent tenant related obligations based on executed leases as of December 31, 2013	89,986
Future minimum lease payments receivable:
2014	589,603
2015	564,468
2016	501,714
2017	441,682
2018	391,015
Thereafter	1,350,719
Total	3,839,201
Future minimum lease payments:
2014	1,477
2015	1,477
2016	1,477
2017	1,477
2018	1,483
Thereafter	131,496
Total	138,887
Ground lease expenses	$ 1,857	$ 1,828	$ 1,850