Basis of Presentation (Details) £ in Millions, $ in Millions		Oct. 23, 2017 USD ($) shares	Oct. 23, 2017 GBP (£) shares	Sep. 30, 2017 USD ($) property	Dec. 31, 2016 USD ($)
Noncontrolling Interest [Line Items]
Assets	[1]			$ 8,536.6	$ 7,659.1
Subsequent Event [Member] | KWE [Member]
Noncontrolling Interest [Line Items]
Cash consideration to shareholders		$ 697.2	£ 528.6
Equity interest issued (in shares) | shares		37,226,695	37,226,695
Properties, Loans and Servicing Platform in Europe [Member] | Europe [Member]
Noncontrolling Interest [Line Items]
Assets				$ 1,028.2
KWE [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent				23.80%	23.60%
KWE [Member] | Equity Securities [Member]
Noncontrolling Interest [Line Items]
Investment, gross cash basis				$ 492.6
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				64.00%
Equity in consolidated investments				$ 295.3

[1] The assets and liabilities as of September 30, 2017 include $4.9 billion (including cash held by consolidated investments of $0.7 billion and real estate and acquired in place lease values, net of accumulated depreciation and amortization of $3.9 billion) and $3.0 billion (including investment debt of $2.7 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.