Significant Accounting Policies - Goodwill (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 99.6us-gaap_Goodwill	$ 0us-gaap_Goodwill
Goodwill acquired during the year	3.3us-gaap_GoodwillAcquiredDuringPeriod	99.6us-gaap_GoodwillAcquiredDuringPeriod
Goodwill allocated to sale of Maui Mall	(0.6)us-gaap_GoodwillOtherChanges
Goodwill, ending balance	102.3us-gaap_Goodwill	99.6us-gaap_Goodwill
Materials & Construction
Goodwill [Roll Forward]
Goodwill, beginning balance	90.3	0
Goodwill acquired during the year	3.3	90.3
Goodwill allocated to sale of Maui Mall	0
Goodwill, ending balance	93.6	90.3
Real Estate Leasing
Goodwill [Roll Forward]
Goodwill, beginning balance	9.3	0
Goodwill acquired during the year	0	9.3
Goodwill allocated to sale of Maui Mall	(0.6)
Goodwill, ending balance	$ 8.7	$ 9.3

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

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- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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Amount of other (increase) decrease of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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