Summary of Significant Accounting Policies (Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Tax Treatment Of Dividend
Internal taxable income distribution policy (in percentage)	100.00%
Differences between book and tax basis	$ 139,364
Reconciliation of Net Income to Estimated Taxable Income
Net income	41,939	$ 60,075	$ 32,844
Straight-line rent adjustments	21,048	2,359	5,870
Depreciation and amortization	2,112	2,751	(6,586)
Change in fair value of marketable securities	8,599	8,757	11,990
Loss from discontinued operations (see Note 6)	0	0	23,797
Other	7,677	137	440
Estimated taxable income	$ 81,375	$ 74,079	$ 68,355
Minimum
Tax Treatment Of Dividend
Real estate investment trust distributable income policy (in percentage)	90.00%
Ordinary Income
Tax Treatment Of Dividend
Payments of dividends net percent	100.00%	99.60%	100.00%
Long Term Capital Gain
Tax Treatment Of Dividend
Payments of dividends net percent		0.40%