Net Gain (Loss) on Disposition of Full or Partial Interest in Rental Properties (Details) (USD $)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Jan. 31, 2012
Net gain (loss) on disposition of partial interest in rental properties and other investments
Net gain (loss) on disposition of full or partial interest in rental properties	$ (146,000)		$ 386,559,000	$ (613,000)	$ 386,559,000
Stapleton - 3055 Roslyn [Member]
Net gain (loss) on disposition of partial interest in rental properties and other investments
Net gain (loss) on disposition of full or partial interest in rental properties	(146,000)		0	(146,000)	0
QIC Joint Venture [Member]
Net gain (loss) on disposition of partial interest in rental properties and other investments
Net gain (loss) on disposition of full or partial interest in rental properties	0		381,627,000	(467,000)	381,627,000
New York Retail Joint Venture [Member]
Net gain (loss) on disposition of partial interest in rental properties and other investments
Net gain (loss) on disposition of full or partial interest in rental properties	$ 0	$ 109,533,000	$ 4,932,000	$ 0	$ 4,932,000	$ 9,561,000