Basis of Presentation (Details) $ / shares in Units, $ in Millions				6 Months Ended
		Jun. 13, 2017 £ / shares shares	Jun. 09, 2017 USD ($)	Jun. 30, 2017 USD ($) property $ / shares	Jun. 30, 2016 USD ($)	Dec. 31, 2017 $ / shares	Apr. 24, 2017 shares	Dec. 31, 2016 USD ($)
Noncontrolling Interest [Line Items]
Assets	[1]			$ 8,284.3				$ 7,659.1
Borrowings under line of credit				$ 400.0	$ 100.0
Dividends payable (per share) | $ / shares				$ 0.17
Forecast [Member]
Noncontrolling Interest [Line Items]
Dividends payable (per share) | $ / shares						$ 0.19
Unsecured Debt [Member] | Revolving Credit Facility [Member] | KW Revolving Facility [Member]
Noncontrolling Interest [Line Items]
Borrowings under line of credit			$ 350.0	$ 400.0
KWE [Member]
Noncontrolling Interest [Line Items]
Cash consideration (per share) | £ / shares		£ 2.50
Properties, Loans and Servicing Platform in Europe [Member] | Europe [Member]
Noncontrolling Interest [Line Items]
Assets				$ 934.2
KWE [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent		75.00%		23.80%			64.00%	23.60%
Shares issued per outstanding share | shares		0.3854					0.667
Cash consideration to shareholders (per share) | £ / shares		£ 3.00
Ownership percentage by noncontrolling owners		25.00%					36.00%
Escrow deposit			$ 18.0
Properties, Loans and Servicing Platform in Europe [Member]
Noncontrolling Interest [Line Items]
Number of real estate investment properties in Europe | property				17
Properties, Loans and Servicing Platform in Europe [Member] | Europe [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent				65.00%
Equity in consolidated investments				$ 264.2
Equity Securities [Member] | KWE [Member]
Noncontrolling Interest [Line Items]
Investment, gross cash basis				$ 492.6

[1] The assets and liabilities as of June 30, 2017 include $4.8 billion (including cash held by consolidated investments of $0.6 billion and real estate and acquired in place lease values, net of accumulated depreciation and amortization of $3.8 billion) and $2.9 billion (including investment debt of $2.6 billion), respectively, from consolidated variable interest entities ("VIEs"). The assets and liabilities as of December 31, 2016 include $4.5 billion (including cash held by consolidated investments of $0.6 billion and real estate and acquired in place lease values, net of accumulated depreciation and amortization of $3.6 billion) and $2.7 billion (including investment debt of $2.4 billion), respectively, from VIEs. These assets can only be used to settle obligations of the consolidated VIEs, and the liabilities do not have recourse to the Company.