Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 17,822us-gaap_NetIncomeLoss	$ 15,244us-gaap_NetIncomeLoss
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization, including amortization of debt issuance costs	8,044	7,898
Straight-lining of rental income	(551)us-gaap_StraightLineRent	(641)us-gaap_StraightLineRent
Change in operating assets and liabilities:
Tenant and other receivables, net	(1,212)	193
Other assets	11,165	9,547
Amounts due to Vornado	(29)	(815)
Accounts payable and accrued expenses	9,861	8,888
Other liabilities	(7)	(3)
Net cash provided by operating activities	45,093	40,311
CASH FLOWS FROM INVESTING ACTIVITIES
Construction in progress and real estate additions	(14,908)	(1,444)
Change in restricted cash	(13)	4,510
Net cash (used in) provided by investing activities	(14,921)	3,066
CASH FLOWS FROM FINANCING ACTIVITIES
Debt repayments	(776)us-gaap_RepaymentsOfLongTermDebt	(314,937)us-gaap_RepaymentsOfLongTermDebt
Proceeds from borrowing	0	300,000
Dividends paid	(17,890)us-gaap_PaymentsOfDividendsCommonStock	(16,607)us-gaap_PaymentsOfDividendsCommonStock
Debt issuance costs	(9)us-gaap_PaymentsOfDebtIssuanceCosts	(4,223)us-gaap_PaymentsOfDebtIssuanceCosts
Net cash used in financing activities	(18,675)	(35,767)
Net increase in cash and cash equivalents	11,497	7,610
Cash and cash equivalents at beginning of period	227,815	347,718
Cash and cash equivalents at end of period	239,312	355,328
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash payments for interest, excluding capitalized interest of $459 and $30, respectively	6,220us-gaap_InterestPaid	10,291us-gaap_InterestPaid
NON-CASH TRANSACTIONS
Liability for real estate additions (including $4,159 due to Vornado in 2015)	13,717alx_NonCashAdditionsToRealEstate	1,750alx_NonCashAdditionsToRealEstate
Write-off of fully amortized and/or depreciated assets	$ 83	$ 10,569

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