Income Taxes - Deferred Tax Assets and Deferred Tax Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Benefit plans	$ 30.7	$ 21.1
Capitalized costs	21.9	24.1
Charitable contribution	0	1.5
Joint ventures and other investments	19.0	13.0
Impairment and amortization	6.7	0.5
Insurance and other reserves	4.2	6.7
Solar credit	4.9	3.5
Other	9.8us-gaap_DeferredTaxAssetsOther	5.4us-gaap_DeferredTaxAssetsOther
Total deferred tax assets	97.2us-gaap_DeferredTaxAssetsGross	75.8us-gaap_DeferredTaxAssetsGross
Deferred tax liabilities:
Tax-deferred gains on real estate transactions	252.5	225.4
Basis differences for property and equipment	19.3	23.4
Straight-line rental income and advanced rent	8.4alex_Fasb13RentIncomeAndAdvancedRent	7.2alex_Fasb13RentIncomeAndAdvancedRent
Other	2.7us-gaap_DeferredTaxLiabilitiesOther	5.2us-gaap_DeferredTaxLiabilitiesOther
Total deferred tax liabilities	282.9us-gaap_DeferredIncomeTaxLiabilities	261.2us-gaap_DeferredIncomeTaxLiabilities
Net deferred tax liability	185.7us-gaap_DeferredTaxLiabilities	185.4us-gaap_DeferredTaxLiabilities
Investment Tax Credit Carryforward \| Tax Year 2014
Deferred tax assets:
Solar credit	$ 3.7

The tax effect as of the balance sheet date of the amount of the estimated future tax deductions arising from capitalized development costs, which can only be realized if sufficient tax-basis income is generated in future periods to enable the deduction to be taken.

No definition available.