Net Gain (Loss) on Disposition of Full or Partial Interest in Rental Properties (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Disposal Group, not including discontinued operations [Line Items]
Net gain (loss) on disposition of full or partial interest in rental properties	$ 1,746,000	$ (146,000)	$ 1,746,000	$ (613,000)
Skylight Office Tower [Member]
Disposal Group, not including discontinued operations [Line Items]
Net gain (loss) on disposition of full or partial interest in rental properties	1,746,000	0	1,746,000	0
Stapleton - 3055 Roslyn [Member]
Disposal Group, not including discontinued operations [Line Items]
Net gain (loss) on disposition of full or partial interest in rental properties	0	(146,000)	0	(146,000)
Other [Member]
Disposal Group, not including discontinued operations [Line Items]
Net gain (loss) on disposition of full or partial interest in rental properties	$ 0	$ 0	$ 0	$ (467,000)