Debt (Credit Facilities Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Revolving Line of Credit [Abstract]
Outstanding borrowings on term loans	$ 508,633,000		$ 508,633,000		$ 547,780,000
Line of Credit					27,000,000
Line of credit facility covenant minimum tangible net worth	$ 900,000,000		$ 900,000,000
Line of credit facility covenant percentage of net cash proceeds of issuance and sales of equity interests (in hundredths)			75.00%
Line of credit facility covenant maximum annual distributions (in hundredths)	95.00%		95.00%
Line of credit facility covenant maximum leverage ratio (in hundredths)	60.00%		60.00%
Interest Rate Swap [Member]
Revolving Line of Credit [Abstract]
Notional amount	$ 150,000,000		$ 150,000,000
Minimum [Member]
Revolving Line of Credit [Abstract]
Line of credit facility covenant fixed charge coverage ratio	1.50		1.50
Maximum [Member]
Revolving Line of Credit [Abstract]
Line of credit facility covenant maximum secured debt leverage ratio (in hundredths)	45.00%		45.00%
$300 Million Senior Unsecured Term Loan Agreement ("Second Term Loan")[Member]
Revolving Line of Credit [Abstract]
Debt instrument, face amount	$ 300,000,000		$ 300,000,000
Maturity date			Aug. 10, 2020
$300 Million Senior Unsecured Term Loan Agreement ("Second Term Loan")[Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)			1.50%
$300 Million Senior Unsecured Term Loan Agreement ("Second Term Loan")[Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)			2.25%
$500 Million Senior Unsecured Credit Agreement ("Credit Facility") [Member]
Revolving Line of Credit [Abstract]
Revolving line of credit, current borrowing capacity	500,000,000		$ 500,000,000
Revolving line of credit, maximum borrowing capacity	850,000,000		$ 850,000,000
Line of credit, expiration date			Feb. 28, 2018
Renewal period of line of credit			1 year
Line of credit, remaining borrowing capacity	246,710,000		$ 246,710,000
$250 Million Senior Unsecured Revolving Line Of Credit ("Line of Credit") [Member]
Revolving Line of Credit [Abstract]
Line of Credit					27,000,000
$250 Million Senior Unsecured Revolving Line Of Credit ("Line of Credit") [Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)			1.70%
$250 Million Senior Unsecured Revolving Line Of Credit ("Line of Credit") [Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)			2.45%
$250 Million Unsecured Term Loan ("First Term Loan") [Member]
Revolving Line of Credit [Abstract]
Revolving line of credit, current borrowing capacity	$ 250,000,000		$ 250,000,000
$250 Million Unsecured Term Loan ("First Term Loan") [Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)			1.60%
Fixed interest rate	2.914%		2.914%
$250 Million Unsecured Term Loan ("First Term Loan") [Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)			2.35%
Revolving Line Of Credit [Member]
Revolving Line of Credit [Abstract]
Revolving line of credit, current borrowing capacity	$ 250,000,000		$ 250,000,000
Description of variable rate basis			one month U.S. LIBOR
Line of credit, financial covenant terms			The Credit Facility and the Second Term Loan agreements include certain financial covenants and require that we maintain: (1) a minimum tangible net worth (calculated as total assets, plus accumulated depreciation, less total liabilities, intangibles and other defined adjustments) of $900,000, plus an amount equal to 75% of the net cash proceeds of all issuances and primary sales of equity interests of the parent guarantor or any of its subsidiaries consummated following the closing date; (2) annual distributions not to exceed 95% of adjusted funds from operations; and (3) certain financial ratios, including the following:· a fixed charge coverage ratio of not less than 1.50 to 1.00;· a maximum leverage ratio of not more than 60%; and· a maximum secured debt leverage ratio of 45%.The Company is in compliance with each of the covenants listed above as of June 30, 2016.
Interest Expense, on credit facilities	$ 4,083,000	$ 2,238,000	$ 8,563,000	$ 3,980,000
Line of credit, weighted average interest rate (in hundredths)	2.78%	2.75%	2.79%	2.76%
$500 Million Senior Unsecured Credit Agreement And $300 Million Senior Unsecured Term Loan Agreement [Member]
Revolving Line of Credit [Abstract]
Deferred costs, net of accumulated amortization	$ 1,887,000		$ 1,887,000		$ 2,220,000