Summary of Significant Accounting Policies (Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Tax Treatment Of Dividend
Internal taxable income distribution policy (in percentage)									100.00%
Differences between book and tax basis	$ 171,331								$ 171,331
Reconciliation of Net Income to Estimated Taxable Income
Net income	$ 14,434	$ 16,493	$ 11,283	$ 17,865	$ 9,971	$ 15,003	$ 17,570	$ (9,700)	60,075	$ 32,844	$ 80,509
Straight-line rent adjustments									2,359	5,870	4,250
Depreciation and amortization timing differences									2,751	(6,586)	3,084
Change in fair value of marketable securities									8,757	11,990	0
Loss from discontinued operations (see Note 6)									0	23,797	0
Other									137	440	(343)
Estimated taxable income									$ 74,079	$ 68,355	$ 87,500
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									99.60%	100.00%	99.50%
Long Term Capital Gain
Tax Treatment Of Dividend
Payments of dividends net percent									0.40%		0.50%