Acquisitions (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended			0 Months Ended
	Jun. 03, 2013 US Airways Building [Member] sqft Property	Apr. 26, 2013 Lincoln Place [Member] sqft Story	Jan. 17, 2013 Tower Place 200 [Member] sqft	Mar. 07, 2013 Deerwood Portfolio [Member] sqft Property	Mar. 25, 2013 Tampa Fund II Assets [Member] Property sqft
Business Acquisition [Line Items]
Area of real estate property (in square feet)	225,000	140,000	258,000	1,000,000	788,000
Cost of entity acquired	$ 41.8		$ 56.3	$ 130.0	$ 97.5
Number of office properties purchased in one transaction				2	3
Mortgage secured for properties in a business acquisition	13.9			84.5
Maturity date of debt		June 11, 2016
Interest rate on mortgage (in percent)	3.00%	5.90%		3.90%
Company purchased its co-investor's interest					70.00%
Ownership percentage of property (in percent)	75.00%
Maturity date	Dec. 01, 2016			Apr. 01, 2023
Number of stories in a property acquired	9
Leased percentage of a property acquisition (in percent)	100.00%	100.00%
Expiration date of lease	Apr. 01, 2024
First termination option	Dec. 31, 2016
Second termination option	Dec. 31, 2021
Months notice of termination	0 years 12 months 0 days
Ownership by noncontrolling owners (in percent)	25.00%				70.00%
Mortgage assumed in a business acquisition		49.6			40.7
Mortgage loan on property in business combination					58.1
Area of office space under contract		111,000
Area of retail space under contract		29,000
Number of stories in parking garage		5
Operating partnership units issued		900,000
closing stock price		$ 17.78
Implied purchase price		$ 65.6