Organization, Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Operating Partnership's Equity Interest (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Fund II and Mervyns II
Variable Interest Entity [Line Items]
Operating Partnership Share of Capital	28.33%
Capital called	$ 347.1
Unfunded Commitment	$ 0.0
Equity Interest Held By Operating Partnership	28.33%
Preferred Return	8.00%
Total distributions	$ 131.6
Fund III
Variable Interest Entity [Line Items]
Operating Partnership Share of Capital	24.54%
Capital called	$ 411.5
Unfunded Commitment	$ 38.5
Equity Interest Held By Operating Partnership	24.54%
Preferred Return	6.00%
Total distributions	$ 551.9
Fund IV
Variable Interest Entity [Line Items]
Operating Partnership Share of Capital	23.12%
Capital called	$ 412.7
Unfunded Commitment	$ 117.3
Equity Interest Held By Operating Partnership	23.12%
Preferred Return	6.00%
Total distributions	$ 131.5
Fund V
Variable Interest Entity [Line Items]
Operating Partnership Share of Capital	20.10%
Capital called	$ 45.8
Unfunded Commitment	$ 474.2
Equity Interest Held By Operating Partnership	20.10%
Preferred Return	6.00%
Total distributions	$ 0.0