Co-investments - Summary of Investments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 1,267,593	$ 1,119,258
Total operating and other co-investments, net [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 840,060	780,332
Membership Interest In CPPIB [Member]
Schedule of Equity Method Investments [Line Items]
Limited partnership interest in partnership investments	54.00%
Co-investments	$ 487,002	500,287
Membership interest in Wesco I, III, IV, and V [Member]
Schedule of Equity Method Investments [Line Items]
Limited partnership interest in partnership investments	52.00%
Co-investments	$ 187,905	214,408
Membership interest in BEXAEW, BEX II And BEX III [Member]
Schedule of Equity Method Investments [Line Items]
Limited partnership interest in partnership investments	50.00%
Co-investments	$ 118,924	13,827
Membership In Other [Member]
Schedule of Equity Method Investments [Line Items]
Limited partnership interest in partnership investments	51.00%
Co-investments	$ 46,229	51,810
Total development co-investments, net [Member]
Schedule of Equity Method Investments [Line Items]
Limited partnership interest in partnership investments	50.00%
Co-investments	$ 77,144	73,770
Total preferred interest co-investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	350,389	265,156
Total preferred interest co-investments [Member] | Investments in Majority-owned Subsidiaries [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments	$ 36,800	15,700
BEX II [Member]
Schedule of Equity Method Investments [Line Items]
Co-investments		$ (36,700)