Net Gain on Change in Control of Interests (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net Gain on Change in Control of Interests [Line Items]
Net gain on change in control of interests	$ 487,684,000	$ 0	$ 487,684,000	$ 2,759,000
University Park at MIT [Member]
Net Gain on Change in Control of Interests [Line Items]
Net gain on change in control of interests	463,643,000	0	463,643,000	0
Cherry Tree [Member]
Net Gain on Change in Control of Interests [Line Items]
Net gain on change in control of interests	7,391,000	0	7,391,000	0
Chestnut Lake [Member]
Net Gain on Change in Control of Interests [Line Items]
Net gain on change in control of interests	8,525,000	0	8,525,000	0
Stratford Crossing [Member]
Net Gain on Change in Control of Interests [Line Items]
Net gain on change in control of interests	8,125,000	0	8,125,000	0
Other Property [Member]
Net Gain on Change in Control of Interests [Line Items]
Net gain on change in control of interests	$ 0	$ 0	$ 0	$ 2,759,000