Investments in Real Estate Entities (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	$ 70,744	$ 64,962	$ 70,018
Gain on sale of communities	136,333	196,749	98,899
Fund I
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	0	87	871
Fund II
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	53,961	49,882	32,211
Equity in income of unconsolidated entities relating to the Company's recognition of promoted interest	26,742	7,985
Gain on sale of communities	26,322	41,501	29,726
US Fund
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	14,773	15,635	2,052
Gain on sale of communities	13,788	16,568
ACJV
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	1,388	1,445	511
MVPI LLC
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	1,833	1,627	22,453
Equity in income of unconsolidated entities relating to the Company's recognition of promoted interest			21,340
MVPI LLC | Joint Venture Agreement Modification [Member]
Summary of the Company's equity in income of unconsolidated entities
Payments for (Proceeds from) Other Real Estate Partnerships			20,680
Brandywine [Member]
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	106	10	(1,474)
CVP I, LLC
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	0	9	1,812
Equity in income of unconsolidated entities relating to the Company's recognition of promoted interest			1,289
Residual JV
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	(1,223)	(1,374)	11,582
Avalon Clarendon [Member]
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	0	(2,359)	0
AVA North Point [Member]
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	(122)	0	0
Sudbury [Member]
Summary of the Company's equity in income of unconsolidated entities
Equity in income of unconsolidated real estate entities	$ 28	$ 0	$ 0