Investment Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015 acquisition Investment	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt	$ 3,375.1us-gaap_LongTermDebt	$ 3,023.3us-gaap_LongTermDebt
Number of acquisitions partially financed with mortgages	4
Number of investments partially financed with mortgages	1
Number of investments with existing mortgages refinanced	7
Mortgage Loans and Notes Payable [Member]
Debt Instrument [Line Items]
Long-term debt	2,672.7
Debt unamortized premium	9.7
Mortgage Loans Payable [Member]
Debt Instrument [Line Items]
Long-term debt	2,672.7	2,195.9
Debt unamortized premium	9.7	15.4
Weighted average interest rate of debt	2.89%	3.03%
Percent of debt with fixed rate	58.00%	43.00%
Percent of debt with floating interest rate and interest caps	25.00%	38.00%
Percent of debt with floating interest rate, without interest caps	17.00%	19.00%
Mortgage Loans Payable [Member] \| Western United States \| Mortgage Loans Secured by Commercial Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	142.8	131.0
Mortgage Loans Payable [Member] \| Western United States \| Mortgage Loans Secured by Multi-Family Properties [Member]
Debt Instrument [Line Items]
Long-term debt	592.9	565.5
Mortgage Loans Payable [Member] \| Western United States \| Mortgage Loans Secured by Hotel [Member]
Debt Instrument [Line Items]
Long-term debt	38.7	37.2
Mortgage Loans Payable [Member] \| Japan \| Mortgage Loans Secured by Commercial Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	2.1	2.1
Mortgage Loans Payable [Member] \| Japan \| Mortgage Loans Secured by Multi-Family Properties [Member]
Debt Instrument [Line Items]
Long-term debt	236.6	242.9
Mortgage Loans Payable [Member] \| Ireland \| Mortgage Loans Secured by Commercial Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	366.3	412.5
Mortgage Loans Payable [Member] \| Ireland \| Mortgage Loans Secured by Multi-Family Properties [Member]
Debt Instrument [Line Items]
Long-term debt	162.3	133.6
Mortgage Loans Payable [Member] \| Ireland \| Mortgage Loans Secured by Residential and Other [Member]
Debt Instrument [Line Items]
Long-term debt	36.3	29.0
Mortgage Loans Payable [Member] \| Ireland \| Mortgage Loans Secured by Hotel [Member]
Debt Instrument [Line Items]
Long-term debt	64.9	72.9
Mortgage Loans Payable [Member] \| United Kingdom \| Mortgage Loans Secured by Commercial Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	1,029.8	569.2
KWE [Member] \| Mortgage Loans Payable [Member] \| Ireland \| Mortgage Loans Secured by Commercial Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	287.1	323.8
KWE [Member] \| Mortgage Loans Payable [Member] \| Ireland \| Mortgage Loans Secured by Multi-Family Properties [Member]
Debt Instrument [Line Items]
Long-term debt	32.2	40.3
KWE [Member] \| Mortgage Loans Payable [Member] \| Ireland \| Mortgage Loans Secured by Residential and Other [Member]
Debt Instrument [Line Items]
Long-term debt	16.7	14.6
KWE [Member] \| Mortgage Loans Payable [Member] \| United Kingdom \| Mortgage Loans Secured by Commercial Buildings [Member]
Debt Instrument [Line Items]
Long-term debt	$ 946.2	$ 483.0
KWE [Member]
Debt Instrument [Line Items]
Ownership percentage by parent	16.00%

Debt Instrument, Percent with Fixed Rate

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- Definition

Debt Instrument, Percent With Floating Rate With Interest Caps

+ References

No definition available.

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- Definition

Debt Instrument, Percent With Floating Rate Without Interest Caps

+ References

No definition available.

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- Definition

Number of Acquisitions Partially Financed with Mortgages

+ References

No definition available.

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- Definition

Number of Investments Partially Financed With Mortgages

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- Definition

Number of Investments with Existing Mortgages Refinanced

+ References

No definition available.

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- Details

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- Definition

The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

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- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20, 22

-Article 5

+ Details

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- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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