Consolidated Statements Of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Cash Flows [Abstract]
Non-cash sale of equity-method investment	$ 0	$ 8,100,000	$ 0
Cash flows from operating activities:
Net income	159,365,000	152,716,000	150,314,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	67,173,000	59,525,000	53,123,000
Amortization	5,790,000	3,563,000	3,472,000
Straight-line rental income	(26,090,000)	(22,198,000)	(24,623,000)
Non-cash interest income on construction loan	(792,000)	(1,021,000)	(411,000)
Unamortized debt premium written off	(353,000)	18,000	(18,000)
Write-off of debt issuance costs	3,592,000	3,514,000	3,413,000
Loan and realty losses (recoveries), net	0	(15,856,000)	491,000
Gain on sale of real estate	(50,000)	(4,582,000)	(1,126,000)
Loss on extinguishment of debt	(2,214,000)	0	0
Gain on purchase liability settlement	0	1,657,000	0
Net realized gains on sales of marketable securities	(10,038,000)	(29,673,000)	(23,529,000)
Share-based compensation	2,612,000	1,732,000	2,134,000
Amortization of commitment fees and note receivable discounts	517,000	693,000	0
Amortization of Lease Incentives	119,000	40,000	40,000
Income from equity-method investee	0	1,214,000	1,767,000
Change in operating assets and liabilities:
Equity-method investment and other assets	(4,372,000)	1,018,000	216,000
Accounts payable and accrued expenses	1,607,000	2,764,000	1,038,000
Deferred income	304,000	(1,385,000)	2,501,000
Net cash provided by operating activities	197,325,000	177,219,000	164,425,000
Cash flows from investing activities:
Investment in mortgage and other notes receivable	(49,853,000)	(92,051,000)	(92,249,000)
Collection of mortgage and other notes receivable	43,168,000	84,228,000	21,495,000
Investment in real estate	(157,214,000)	(359,257,000)	(106,315,000)
Investment in real estate development	(10,691,000)	(32,102,000)	(14,641,000)
Investment in renovations of existing real estate	(7,888,000)	(3,378,000)	(3,157,000)
Payment allocated to cancellation of lease purchase option	0	6,400,000	0
Payment of real estate purchase liability	0	(8,208,000)	0
Proceeds from disposition of real estate properties	450,000	27,723,000	9,593,000
Purchases of marketable securities	0	0	(8,458,000)
Proceeds from sales of marketable securities	18,182,000	59,607,000	57,406,000
Net cash used in investing activities	163,846,000	329,838,000	136,326,000
Cash flows from financing activities:
Net change in borrowings under revolving credit facilities	63,000,000	124,000,000	(340,000,000)
Proceeds from issuance of secured debt	0	0	78,084,000
Borrowings on term loan	250,000,000	75,000,000	325,000,000
Payments of term loans	(250,822,000)	(767,000)	(742,000)
Debt issuance costs	(4,935,000)	(258,000)	(2,608,000)
Taxes remitted in relation to employee stock options exercised	571,000	1,133,000	0
Proceeds from equity offering, net	122,237,000	104,190,000	49,114,000
Convertible bond redemption	(60,921,000)	0	0
Proceeds from exercise of stock options	0	(1,000)	(1,000)
Distributions to noncontrolling interest	0	(1,565,000)	(2,292,000)
Distribution to acquire non-controlling interest	0	17,000,000	0
Dividends paid to stockholders	(153,040,000)	(138,303,000)	(124,657,000)
Net cash provided by (used in) financing activities	(35,052,000)	144,165,000	(18,100,000)
(Decrease) increase in cash and cash equivalents	(1,573,000)	(8,454,000)	9,999,000
Cash and cash equivalents, beginning of year	4,636,000	13,090,000	3,091,000
Cash and cash equivalents, end of year	3,063,000	4,636,000	13,090,000
Supplemental disclosure of cash flow information:
Interest Paid	45,405,000	39,539,000	31,289,000
Supplemental schedule of non-cash investing and financing activities:
Settlement of mortgage note by real estate acquisition	0	0	1,545,000
Tax deferred exchange funds applied to investment in real estate	0	(227,000)	0
Conditional consideration in asset acquisition	0	0	(3,000,000)
Settlement of contingent asset acquisition liability	0	0	750,000
Accounts payable increase due to investments in real estate	(1,855,000)	(430,000)	(1,076,000)
Tenant investment in leased asset	1,250,000	0	0
Reclass of note balance into real estate investment	0	9,753,000	255,000
Assumption of debt in real estate acquisition	18,311,000	0	0
Conversion of investment in preferred stock to common	0	0	38,132,000
Unsettled marketable securities sales transactions	$ 0	$ 6,464,000	$ 0