Investments in Joint Ventures (Details Textual) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013 sqft	Jun. 30, 2012	Jun. 30, 2013 sqft	Jun. 30, 2012	Dec. 31, 2012
Business Acquisition Area of Land	1,600,000		1,600,000
Investment in joint ventures	$ 49,920		$ 49,920		$ 72,742
Net Income (loss)	(4,894)	(19,711)	390,283	(20,070)
Percentage Of Land Leased	100.00%		100.00%
Mortagage Notes Payable	0		0		312,140
Proceeds From Divestiture Of Interest In Joint Venture			8,275	0
Income (Loss) From Equity Method Investments	(2,603)	29	(3,791)	57
Proceeds From Distributions Received From Joint Venture	21,381		31,248
Interest Expense, Total	85	0	85	0
Number of Properties acquired	20		14
Proceeds From Interest In Joint Venture			3,600
Bank Of America [Member]
Equity Method Investment, Ownership Percentage	50.00%		50.00%
Land Subject to Ground Leases	3,600,000		3,600,000
Portfolio Share Percentage			93.00%
Mortgage Loan To Finance Joint Venture Acquisition			200,000
Real Estate Investments, Joint Ventures	49,865		49,865		72,541
Number Of Realty Properties	88		88
Lease Expiration Description			expiration dates through 2023
Percentage Of Land Leased	90.00%		90.00%
Proceeds From Divestiture Of Interest In Joint Venture	19,325		20,575
Interest Expense, Total	2,849		7,092
Philips Building [Member]
Noncontrolling Interest, Ownership Percentage by Parent	25.00%		25.00%
Business Acquisition Area of Land	200,000		200,000
Percentage Of Net Leased To Related Party	100.00%		100.00%
Investment in joint ventures	55		55		201
Net Income (loss)	31		61
Mortgage Loan To Finance Joint Venture Acquisition			41,000
Income (Loss) From Equity Method Investments		29		57
Proceeds From Distributions Received From Joint Venture	$ 103		$ 206