Summary of Significant Accounting Policies (Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Tax Treatment Of Dividend
Internal Taxable Income Distribution Policy (in percentage)									100.00%
Differences Between Book and Tax Basis	$ 208,622								$ 208,622
Reconciliation of Net Income to Estimated Taxable Income
Net income	$ 21,655	$ 21,036	$ 21,767	$ 22,019	$ 23,572	$ 18,172	$ 17,341	$ 17,822	86,477	$ 76,907	$ 67,925
Straight-lining of rental income									2,347	(1,418)	(2,538)
Depreciation and amortization timing differences									(14,534)	2,477	2,283
Reversal of income tax liability									0	0	(420)
Other									2,975	751	765
Estimated taxable income									$ 77,265	$ 78,717	$ 68,015
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									97.70%	97.30%	100.00%
Long Term Capital Gain
Tax Treatment Of Dividend
Payments Of Dividends Net Percent									2.30%	2.70%	0.00%