Unconsolidated Investments - Narrative (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2018 USD ($) limited_partnership joint_venture	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Income from unconsolidated investments				$ 36,900,000	$ 35,900,000
Equity method investment distribution received				24,700,000
Other assets	$ (253,400,000)			(253,400,000)		$ (263,100,000)
Income from unconsolidated investments	10,900,000	$ 10,800,000		36,900,000	39,800,000
Unconsolidated investments	613,700,000			613,700,000		519,300,000
Payments to acquire equity method investments				125,600,000	36,700,000
Proceeds from equity method investments				$ 67,300,000
Number of joint ventures with unfulfilled capital commitments | joint_venture				4
Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Unfunded capital commitments	45,300,000			$ 45,300,000
Vintage Housing Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Number of syndicated limited partnerships (over) | limited_partnership				43
Equity method investments	101,000,000			$ 101,000,000		114,800,000
Income from unconsolidated investments	3,900,000	1,500,000		15,700,000	15,600,000
Gain on fair value through equity income	2,300,000	0		12,100,000	12,400,000
Equity method investment distribution received	$ 1,600,000	1,500,000		3,600,000	3,200,000
Axa Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Payments to acquire equity method investments				89,800,000
Kennedy Wilson Real Estate Fund VI, LP [Member]
Schedule of Equity Method Investments [Line Items]
Payments to acquire equity method investments				$ 12,500,000
Minimum [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	5.00%			5.00%
Maximum [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	50.00%			50.00%
Accounting Standards Update 2014-09 [Member]
Schedule of Equity Method Investments [Line Items]
Income from unconsolidated investments	$ 6,900,000			$ 17,200,000
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Schedule of Equity Method Investments [Line Items]
Other assets						32,900,000
Income from unconsolidated investments		(2,700,000)	$ (2,700,000)		3,900,000
Unconsolidated investments						$ 32,900,000
Investment management, property services and research fees (includes $3.43, $0.3, $6.3 and $10.0 of related party fees)		$ (2,600,000)			$ 3,900,000