Debt (Credit Facilities Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) agreement	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) agreement	Jun. 30, 2018 USD ($)
Short-term Debt [Line Items]
Number of unsecured credit agreements | agreement	3		3
Debt instrument, face amount	$ 950,900,000		$ 950,900,000
Line of credit facility covenant minimum tangible net worth	$ 1,075,000,000		$ 1,075,000,000
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%
Line of credit facility covenant maximum secured debt leverage ratio (in hundredths)	45.00%		45.00%
Minimum [Member]
Short-term Debt [Line Items]
Line of credit facility covenant fixed charge coverage ratio	1.50		1.50
Credit Facility [Member]
Short-term Debt [Line Items]
Revolving line of credit, current borrowing capacity	$ 457,000,000		$ 457,000,000
Line of credit, expiration date			Aug. 01, 2022
Renewal period of line of credit			1 year
Revolving line of credit, maximum borrowing capacity	857,000,000		$ 857,000,000
Revolving Line Of Credit [Member]
Short-term Debt [Line Items]
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 facility, covenant terms			The Credit Facility and the Term Loans 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 $1,075,000, 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 all of the covenants as of June 30, 2019.
Interest expense, on credit facilities	$ 8,903,000	$ 7,661,000	$ 17,539,000	$ 14,773,000
Line of credit, weighted average interest rate (in hundredths)	4.17%	3.73%	4.15%	3.69%
$250 Million Unsecured Term Loan ("First Term Loan") [Member]
Short-term Debt [Line Items]
Debt instrument, face amount	$ 207,000,000		$ 207,000,000
$300 Million Senior Unsecured Term Loan Agreement ("Second Term Loan")[Member]
Short-term Debt [Line Items]
Debt instrument, face amount	300,000,000		$ 300,000,000
Maturity date			Aug. 10, 2020
$200 Million Senior Unsecured Term Loan Agreement ("Third Term Loan")[Member]
Short-term Debt [Line Items]
Debt instrument, face amount	$ 193,900,000		$ 193,900,000
Maturity date			Aug. 02, 2021