Condensed Consolidated Statements Of Cash Flows (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net income	$ 18,350,000	$ 19,093,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	3,365,000	2,884,000
Straight-line rental income	(1,062,000)	(910,000)
Non-cash interest income on construction loan	0	(221,000)
Gain on sale of real estate	0	(2,299,000)
Net realized gains and recoveries on sales of marketable securities	0	(154,000)
Share-based compensation	1,432,000	2,371,000
Change in fair value of interest rate swap agreement	0	(1,254,000)
Increase (Decrease) in Operating Capital [Abstract]
Accounts receivable	(189,000)	(771,000)
Deferred costs and other assets	56,000	(740,000)
Accounts payable and accrued expenses	(665,000)	(606,000)
Deferred income	(40,000)	(38,000)
Net cash provided by operating activities	21,247,000	17,355,000
Net Cash Provided by (Used in) Investing Activities [Abstract]
Investment in mortgage notes receivable	(52,000)	(2,496,000)
Collection of mortgage notes receivable	871,000	899,000
Investment in real estate properties	(4,881,000)	(326,000)
Receipt of earnest money deposit	0	525,000
Proceeds from disposition of real estate properties	0	5,771,000
Proceeds from sales of marketable securities	0	1,647,000
Net cash used in investing activities	(4,062,000)	6,020,000
Net Cash Provided by (Used in) Financing Activities [Abstract]
Net change in borrowings under a revolving credit facility	(2,000,000)	(37,765,000)
Borrowings on term loan	0	50,000,000
Principal payments on debt	0	(625,000)
Stock options exercised	0	200,000
Dividends paid to stockholders	(24,144,000)	(16,752,000)
Net cash used in financing activities	(26,144,000)	(4,942,000)
(Decrease) increase in cash and cash equivalents	(8,959,000)	18,433,000
Cash and cash equivalents, beginning of year	15,886,000	2,664,000
Cash and cash equivalents, end of year	6,927,000	21,097,000
Supplemental disclosure of cash flow information:
Interest Paid	534,000	660,000
Discount [Member]
Supplemental schedule of non-cash investing and financing activities:
Discount on Mortgage Note Receivable Purchased	$ 0	$ 3,475,000