Co-investments - Summary of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 1,044,208	$ 1,042,423
Total operating co investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	730,151	781,158
Membership interest in Wesco I [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	156,024	188,853
Partnership interest in Fund II [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	228	696
Membership interest in a limited liability company that owns and is developing Expo [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	6,825	7,352
Membership interest in a limited liability company that owns The Huxley [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	0	11,471
Membership Interest In Limited Liability Company That Owns Connolly Station [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	339,600	336,977
Membership interest in Wesco IV [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	65,720	67,937
Membership interest in BEXAEW [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	92,260	97,686
Membership Interest In Palm Valley [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	69,494	70,186
Total development co investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	194,442	152,574
Total preferred interest investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 119,615	$ 108,691