KWE Unsecured Bonds - Schedule of KWE Unsecured Bonds (Details) - Unsecured Bonds [Member] - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 1,295.0	$ 1,325.9
Unamortized premium (discount)	(4.0)	(4.3)
KWE [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	1,299.9	1,331.3
Unamortized loan fees	(4.9)	(5.4)
Long-term debt	1,295.0	1,325.9
KWE [Member] \| KWE Euro Medium Term Note Programme [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	639.7	655.7
KWE [Member] \| United Kingdom [Member] \| KWE Bonds [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	$ 660.2	$ 675.6

Long-term Debt, Net of Premium (Discount)

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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