Discontinued Operations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012 Property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of industrial properties			2alex_NumberOfIndustrialProperties
Number of leased fee properties			2alex_NumberOfLeasedFeeProperties
Gain on sale of income-producing properties, net	$ 0us-gaap_GainLossOnDispositionOfAssets	$ 0us-gaap_GainLossOnDispositionOfAssets	$ 7.3us-gaap_GainLossOnDispositionOfAssets
Income from discontinued operations	34.3	22.3	12.8
Basic Earnings Per Share (USD per share)	$ 0.70	$ 0.50	$ 0.30
Diluted Earnings Per Share (USD per share)	$ 0.70	$ 0.50	$ 0.30
Real Estate Leasing
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from the sale of income-producing properties	70.1	337.6	8.9
Real Estate Leasing revenue	0.3	31.6	36.4
Gain on sale of income-producing properties, net	55.9	22.1	4.0
Real Estate Leasing operating profit	0.3	14.6	17.1
Total operating profit before taxes	56.2	36.7	21.1
Income tax expense	21.9	14.4	8.3
Income from discontinued operations	$ 34.3	$ 22.3	$ 12.8
Industrial \| 2013 Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of real estate properties		4
Retail \| 2014 Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of real estate properties	1
Retail \| 2013 Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of real estate properties		3
Office \| 2013 Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of real estate properties		2

Number of industrial properties.

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Number of leased fee properties.

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The cash inflow from the sale of productive assets related to a discontinued operations.

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Overall income (loss) from a disposal group that is classified as a component of the entity, before income tax, reported as a separate component of income before extraordinary items. Includes the following (before income tax): income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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Tax effect allocated to a disposal group that is classified as a component of the entity reported as a separate component of income before extraordinary items. Includes the tax effects of the following: income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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Amount of operating income or loss attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

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Amount of sales or other form of revenues attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

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Amount of gain (loss) on sale or disposal of property, plant and equipment assets, excluding oil and gas property and timber property.

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Amount of income (loss) from a disposal group, net of income tax, reported as a separate component of income before extraordinary items after deduction or consideration of the amount allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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Per basic share amount, after tax, of income (loss) from the day-to-day business activities of the discontinued operation and gain (loss) from the disposal of the discontinued operation.

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Per diluted share amount, after tax, of income (loss) from the day-to-day business activities of the discontinued operation and gain (loss) from the disposal of the discontinued operation.

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The number of real estate properties owned as of the balance sheet date.

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