Investments in and Advances to Unconsolidated Affiliates - Fund Investments (Details) - USD ($) $ in Millions	1 Months Ended				3 Months Ended
	Feb. 28, 2017	Jan. 31, 2017	Dec. 31, 2016	Jan. 31, 2016	Mar. 31, 2017
Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Repayments of debt					$ 3.5
Fund III [Member] | Cortlandt Towne Center [Member]
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%
Fund III [Member] | Cortlandt Towne Center [Member] | Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Repayments of debt			$ 32.6
Partnership Interest [Member] | Fund III [Member] | Cortlandt Towne Center [Member]
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal			$ 8.8
2819 Kennedy Boulevard [Member] | Fund IV [Member]
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 [Member] | Fund IV [Member] | Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Repayments of debt		8.4
2819 Kennedy Boulevard [Member] | Affiliated Entity [Member] | Fund IV [Member]
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal		6.2
2819 Kennedy Boulevard [Member] | Partnership Interest [Member] | Fund IV [Member]
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal		$ 1.4
Arundel Plaza [Member] | Fund III [Member]
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 [Member] | Fund III [Member] | Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Repayments of debt	10.0
Arundel Plaza [Member] | Affiliated Entity [Member] | Fund III [Member]
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal	5.3
Arundel Plaza [Member] | Partnership Interest [Member] | Fund III [Member]
Schedule of Equity Method Investments [Line Items]
Gain (loss) on disposal	$ 1.3