Summary of Significant Accounting Policies (Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax Treatment Of Dividend
Internal taxable income distribution policy (in percentage)									100.00%
Differences between book and tax basis	$ 199,268								$ 199,268
Reconciliation of Net Income to Estimated Taxable Income
Net income	$ 17,883	$ 20,299	$ 20,660	$ 21,667	$ 21,655	$ 21,036	$ 21,767	$ 22,019	80,509	$ 86,477	$ 76,907
Straight-line rent adjustments									4,250	2,347	(1,418)
Depreciation and amortization timing differences									3,084	(14,534)	2,477
Other									(343)	2,975	751
Estimated taxable income									$ 87,500	$ 77,265	$ 78,717
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									99.50%	97.70%	97.30%
Long Term Capital Gain
Tax Treatment Of Dividend
Payments of dividends net percent									0.50%	2.30%	2.70%