Co-investments - Summary of Investments (Details) (USD $)	3 Months Ended	1 Months Ended
	Mar. 31, 2015	Jul. 31, 2014	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 1,057,298,000		$ 1,042,423,000
Total operating co investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	595,840,000		613,802,000
Membership interest in Wesco I [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	186,667,000		188,853,000
Partnership interest in Fund II [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	232,000		696,000
Membership interest in a limited liability company that owns and is developing Expo [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	6,877,000		7,352,000
Membership interest in a limited liability company that owns The Huxley [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	0		11,471,000
Membership Interest In Limited Liability Company That Owns Connolly Station [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	170,501,000		169,621,000
Membership interest in Wesco IV [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	67,904,000		67,937,000
Membership interest in BEXAEW [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	93,842,000		97,686,000
Membership Interest In Palm Valley [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	69,817,000		70,186,000
Total development co investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	354,384,000		319,930,000
Membership interests in limited liability companies that own and are developing Epic, Lync, Elkhorn, and Folsom and Fifth [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	283,785,000		268,016,000
Membership interests in limited liability companies that owns and are developing One South Market [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	31,300,000		30,919,000
Membership interests in limited liability companies that own and are developing The Huxley and The Dylan [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	0		7,874,000
Membership interests in limited liability companies that own and are developing Century Towers [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	13,475,000		13,121,000
Membership interests in limited liability companies that own and are developing 500 Folsom [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	25,824,000		0
Total preferred interest investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	107,074,000		108,691,000
Preferred interests in related party limited liability companies that owns Sage at Cupertino with a preferred return of 9.5% [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	16,675,000		16,571,000
Preferred stock, stated interest percentage	9.50%
Preferred interest in a related party limited liability company that owns Madison Park at Anaheim with a preferred return of 9% [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	13,824,000		13,824,000
Preferred stock, stated interest percentage	9.00%
Preferred interest in related party limited liability company that owns an apartment development in Redwood City with a preferred return of 12% [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	0		10,396,000
Preferred stock, stated interest percentage	12.00%
Non-operating distributions from co-investment	11,100,000
Preferred interest in related party limited liability company that owns an apartment development in San Jose with a preferred return of 12% [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	10,314,000		10,011,000
Preferred stock, stated interest percentage	12.00%
Extension term	1 year
Number of extension options	1
Preferred interest in a limited liability company that owns 8th & Thomas with a preferred return of 10 [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	13,477,000		13,145,000
Preferred stock, stated interest percentage	10.00%
Extension term	1 year
Number of extension options	1
Preferred interest in a limited liability company that owns Newbury Park with preferred return of 12 [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	13,548,000		13,150,000
Preferred stock, stated interest percentage	12.00%
Preferred interest in a limited liability company that owns Century Towers with preferred return of 10 [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	19,528,000		12,357,000
Preferred Interest In A Limited Liability Company That Owns An Apartment Development In San Jose With A Preferred Return Of Nine Percent [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	19,708,000		19,237,000
Preferred stock, stated interest percentage	9.00%
Century Towers [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 19,000,000	$ 27,000,000
Effective preferred return rate		8.10%