Segment Information - Schedule of Property Level Information by Operating Segment for Properties Classified as Held and Used in Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014 sqft	Sep. 30, 2013 sqft	Sep. 30, 2014 sqft	Sep. 30, 2013 sqft
Segment Reporting Information [Line Items]
Square feet	42,919,000	42,867,000	42,919,000	42,867,000
Rental income	$ 174,216	$ 170,908	$ 518,663	$ 592,221
Tenant reimbursements and other income	42,379	43,293	130,386	146,780
NOI	117,203	111,851	355,225	428,416
CBD Properties
Segment Reporting Information [Line Items]
Square feet	21,892,000	21,844,000	21,892,000	21,844,000
Rental income	109,822	108,624	326,155	330,395
Tenant reimbursements and other income	27,653	28,928	86,359	90,883
NOI	71,596	68,552	216,762	223,066
Suburban Properties
Segment Reporting Information [Line Items]
Square feet	21,027,000	21,023,000	21,027,000	21,023,000
Rental income	64,394	62,284	192,508	261,826
Tenant reimbursements and other income	14,726	14,365	44,027	55,897
NOI	$ 45,607	$ 43,299	$ 138,463	$ 205,350