Debt (Credit Facilities Narrative) (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) agreement	Dec. 31, 2018 USD ($)	Aug. 10, 2017 USD ($)	Oct. 07, 2016 USD ($)
Line of Credit Facility [Abstract]
Outstanding balance, term loans		$ 697,183,000	$ 698,202,000
Line of Credit		48,000,000	10,000,000
Deferred costs, net of accumulated amortization		3,717,000	2,698,000
Line of credit facility covenant minimum tangible net worth		$ 1,119,500,000
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%
Line of credit facility covenant maximum leverage ratio (in hundredths)		60.00%
Line of credit facility covenant maximum secured debt leverage ratio (in hundredths)		45.00%
Line of credit, financial 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%
Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Number of unsecured credit agreements | agreement		3
Debt instrument, face amount		$ 950,900,000
Interest expense on credit facilities	$ 24,066,000	$ 33,563,000	$ 31,189,000
Line of credit, weighted average interest rate (in hundredths)	3.36%	4.11%	3.83%
Minimum [Member]
Line of Credit Facility [Abstract]
Line of credit facility covenant fixed charge coverage ratio		1.50
$475 Million Senior Unsecured Credit Facility ("Credit Facility") [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Line of credit, current borrowing capacity		$ 457,000,000
Line of credit, expiration date		Aug. 10, 2022
Renewal period of line of credit		1 year
$250 Million Senior Unsecured Revolving Line Of Credit ("Line of Credit") [Member]
Line of Credit Facility [Abstract]
Line of Credit		$ 48,000,000	$ 10,000,000
$250 Million Senior Unsecured Revolving Line Of Credit ("Line of Credit") [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Revolving line of credit, maximum borrowing capacity				$ 250,000,000
Line of credit, current borrowing capacity		250,000,000
$225 Million Senior Unsecured Term Loan ("First Term Loan") [Member]
Line of Credit Facility [Abstract]
Outstanding balance, term loans		207,000,000	207,000,000
$225 Million Senior Unsecured Term Loan ("First Term Loan") [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Debt instrument, face amount		207,000,000		$ 225,000,000
Expandable Senior Unsecured Credit Facility ("Credit Facility") [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Revolving line of credit, maximum borrowing capacity		857,000,000
$300 Million Senior Unsecured Term Loan ("Second Term Loan") [Member]
Line of Credit Facility [Abstract]
Outstanding balance, term loans		300,000,000	300,000,000
$300 Million Senior Unsecured Term Loan ("Second Term Loan") [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Debt instrument, face amount		$ 300,000,000
Maturity date		Sep. 10, 2024
$200 Million Senior Unsecured Term Loan ("Third Term Loan") [Member]
Line of Credit Facility [Abstract]
Outstanding balance, term loans		$ 193,900,000	193,900,000
$200 Million Senior Unsecured Term Loan ("Third Term Loan") [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Debt instrument, face amount					$ 193,900,000
Maturity date		Aug. 02, 2021
Term Loans [Member]
Line of Credit Facility [Abstract]
Outstanding balance, term loans		$ 700,900,000	$ 700,900,000