Co-investments - Summary of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Aug. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 1,036,043	$ 20,400	$ 1,042,423
Total operating co investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	718,616		781,158
Membership Interest In Limited Liability Company That Owns Connolly Station [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	335,493		336,977
Membership interest in Wesco I, III, and IV [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	218,316		256,790
Membership interest in BEXAEW [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	89,236		97,686
Membership Interest In Palm Valley [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	69,428		70,186
Partnership interest in Fund II [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	6,143		19,519
Total development co investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	212,892		152,574
Total preferred interest investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	104,535		108,691
Total preferred interest investments [Member] | Investments in Majority-owned Subsidiaries [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 35,400		$ 40,800