Investments in and Advances to Unconsolidated Affiliates - Fund Investments (Details) - USD ($) $ in Millions	1 Months Ended
	Jun. 30, 2017	Feb. 28, 2017	Jan. 31, 2017	Dec. 31, 2016	Jan. 31, 2016
Cortlandt Towne Center
Schedule of Equity Method Investments [Line Items]
Consideration received				$ 57.8	$ 107.3
Proceeds from sale of equity method investments				25.2
Gain (loss) on disposal				$ 36.0	$ 65.4
Percentage of disposition				35.00%	65.00%
Cortlandt Towne Center | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt				$ 32.6
Partnership Interest | Cortlandt Towne Center
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal				$ 8.8
2819 Kennedy Boulevard | Fund IV
Schedule of Equity Method Investments [Line Items]
Consideration received			$ 19.0
Proceeds from sale of equity method investments			10.6
Gain (loss) on disposal			6.3
2819 Kennedy Boulevard | Fund IV | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt			8.4
2819 Kennedy Boulevard | Affiliated Entity | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal			6.2
2819 Kennedy Boulevard | Partnership Interest | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal			$ 1.4
Arundel Plaza | Fund III
Schedule of Equity Method Investments [Line Items]
Consideration received		$ 28.8
Proceeds from sale of equity method investments		18.8
Gain (loss) on disposal		8.2
Arundel Plaza | Fund III | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt		10.0
Arundel Plaza | Affiliated Entity | Fund III
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal		5.3
Arundel Plaza | Partnership Interest | Fund III
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal		$ 1.3
1701 Belmont Avenue | Fund IV
Schedule of Equity Method Investments [Line Items]
Consideration received	$ 5.6
Proceeds from sale of equity method investments	2.7
Gain (loss) on disposal	3.3
1701 Belmont Avenue | Fund IV | Mortgages
Schedule of Equity Method Investments [Line Items]
Repayments of debt	2.9
1701 Belmont Avenue | Affiliated Entity | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal	3.3
1701 Belmont Avenue | Partnership Interest | Fund IV
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal	$ 0.8