Subsequent Events - Narrative (Details) $ in Millions							1 Months Ended	3 Months Ended
	Aug. 04, 2015 USD ($) ft² building	Aug. 03, 2015 USD ($)	Jul. 31, 2015 USD ($) claim	Jul. 24, 2015 USD ($)	Jul. 17, 2015 USD ($)	May. 01, 2015 USD ($)	Jan. 31, 2013 building	Jun. 30, 2015 ft² building	Jun. 30, 2014 ft²
Subsequent Event [Line Items]
Number of buildings sold | building							18
Property square feet | ft²								29,385,000	42,920,000
5.75% unsecured senior notes due 2015
Subsequent Event [Line Items]
Amount defeased						$ 138.8
Disposed of by Sale
Subsequent Event [Line Items]
Number of buildings sold | building								83
Property square feet | ft²								10,834,604
Subsequent event
Subsequent Event [Line Items]
Property square feet | ft²	2,090,162
Gross proceeds from sale of real estate anticipated	$ 376.0
Number of previously disclosed actions | claim			2
Payments for legal settlement			$ 0.2	$ 0.1	$ 0.1
Subsequent event | 111 East Wacker Drive [Member] | Mortgage notes
Subsequent Event [Line Items]
Amount defeased		$ 141.4
Subsequent event | Disposed of by Sale | Illinois Center [Member]
Subsequent Event [Line Items]
Number of buildings sold | building	2