Net Gain on Change in Control of Interests Net Gain on Change in Control of Interests (Details 2) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Business Acquisition [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]
Business Acquisition [Line Items]
Net gain on change in control of interests				463,643,000
Carrying value of previously held equity interests				(62,534,000)
Fair value of previously held equity interests	[1]			401,109,000
Cash paid				386,156,000
Total consideration				$ 787,265,000

[1] The significant assumptions used to value the previously held equity interests in the MIT Assets were determined to be Level 3 inputs. The weighted-average discount rate applied to cash flows and the weighted-average terminal capitalization rate were 7.0% and 6.0%, respectively.