Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended		12 Months Ended							1 Months Ended	12 Months Ended	1 Months Ended	0 Months Ended					1 Months Ended							0 Months Ended
	Jan. 31, 2013 property sqft	Apr. 30, 2012	Dec. 31, 2013 sqft	Dec. 31, 2012 sqft	Feb. 28, 2014 property	Dec. 31, 2013 Suburban property property	Dec. 31, 2012 Suburban property property	Dec. 31, 2013 CBD property	Dec. 31, 2012 CBD property	Jun. 30, 2013 Sale	Dec. 31, 2013 Sale item property sqft	Nov. 30, 2013 Sale CBD and suburban properties property sqft	Dec. 30, 2013 Sale Suburban property property sqft	Oct. 30, 2013 Sale Suburban property property sqft	Oct. 29, 2013 Sale Suburban property property sqft	Jun. 29, 2013 Sale Suburban property property sqft	Jun. 28, 2013 Sale Suburban property property sqft	Dec. 31, 2013 Sale Suburban property property sqft	Oct. 31, 2013 Sale Suburban property property sqft	Aug. 31, 2013 Sale Suburban property property sqft	Jun. 30, 2013 Sale Suburban property property sqft	May 31, 2013 Sale Suburban property property sqft	Apr. 30, 2013 Sale Suburban property property sqft	Jan. 31, 2013 Sale Suburban property property sqft	Mar. 31, 2013 Sale CBD
Real estate properties information
Real estate improvements			$ 122,927
Aggregate sale price of properties sold, excluding closing costs										2,551	249,132	50,500	89,000	13,900	4,000	1,600	5,250	2,020	43,000	4,100	16,300	4,025	830	10,250	1,806
Property square feet			37,273,000	61,806,000							8,343,955	2,125,278	1,670,104	665,545	199,418	30,105	143,802	77,394	1,211,536	129,452	356,045	57,250	618,000	1,060,026
Number of land parcels sold											2
Mortgage financing related to suburban property sold	7,688	419	7,688	1,419																				7,688
Interest rate on real estate mortgage receivable (as a percent)	6.00%	6.00%																						6.00%
Net gains on sale of properties			$ 1,596							$ 1,765			$ (25,521)			$ 317								$ 1,277	$ 1,596
Number of Properties Sold	3										43	18	3					2	9					3
Number of Real Estate Properties Held for Sale					45	42	39	3	1
Number of buildings sold	18										134	21	40	1	2	1	1	2	41	3	2	1	1	18
Number of buildings held for sale						106	93	4	1
Property square feet, held for sale			8,425,548	6,673,851