Summary of Significant Accounting Policies (Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Tax Treatment Of Dividend
Internal taxable income distribution policy (in percentage)									100.00%
Differences between book and tax basis	$ 186,559								$ 186,559
Reconciliation of Net Income to Estimated Taxable Income
Net income	$ 9,971	$ 15,003	$ 17,570	$ (9,700)	$ 17,883	$ 20,299	$ 20,660	$ 21,667	32,844	$ 80,509	$ 86,477
Straight-line rent adjustments									5,870	4,250	2,347
Depreciation and amortization timing differences									(6,586)	3,084	(14,534)
Change in fair value of marketable securities (see Note 5)									11,990	0	0
Loss from discontinued operations (see Note 6)									23,797	0	0
Other									440	(343)	2,975
Estimated taxable income									$ 68,355	$ 87,500	$ 77,265
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%	99.50%	97.70%
Long Term Capital Gain
Tax Treatment Of Dividend
Payments of dividends net percent										0.50%	2.30%