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Debt - Line of Credit (Details) - USD ($)	6 Months Ended
Debt - Line of Credit (Details) - USD ($)	Jun. 30, 2019	Jun. 03, 2019	Dec. 31, 2018	Dec. 21, 2018	Dec. 03, 2018
$250.0 million 5 year - swapped to fixed rate \| Unsecured Debt \| Interest Rate Swap One
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 250,000,000.0
Debt instrument, term	5 years
Unsecured term loans	$ 90,000,000		$ 90,000,000
Notional amount	$ 90,000,000
Interest rate (percent)	2.551%		2.551%
$250.0 million 5 year - swapped to fixed rate \| Unsecured Debt \| Interest Rate Swap Two
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 250,000,000.0
Debt instrument, term	5 years
Unsecured term loans	$ 60,000,000		$ 60,000,000
Notional amount	$ 60,000,000
Interest rate (percent)	2.5525%		2.5525%
$250.0 million 5 year - variable-rate \| Unsecured Debt
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 250,000,000.0
Debt instrument, term	5 years
Unsecured term loans	$ 50,000,000		$ 50,000,000
Interest rate (percent)	3.64%		3.5493%
$250.0 million 5 year - variable-rate \| Unsecured Debt \| Interest Rate Swap One \| One-month LIBOR
Line of Credit Facility [Line Items]
Variable rate		1.20%			1.20%
$250.0 million 5 year - variable-rate \| Unsecured Debt
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 250,000,000.0
Debt instrument, term	5 years
Unsecured term loans	$ 26,000,000		$ 26,000,000
Interest rate (percent)	3.64%		3.6794%
$250.0 million 5 year - variable-rate \| Unsecured Debt \| Interest Rate Swap One \| One-month LIBOR
Line of Credit Facility [Line Items]
Variable rate				1.20%
$150.0 million 5.5 year - variable-rate \| Unsecured Debt
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 150,000,000.0
Debt instrument, term	5 years 6 months
Unsecured term loans	$ 100,000,000		$ 100,000,000
Interest rate (percent)	3.64%		3.5493%
$150.0 million 5.5 year - variable-rate \| Unsecured Debt
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 150,000,000.0
Debt instrument, term	5 years 6 months
Unsecured term loans	$ 26,000,000		$ 26,000,000
Interest rate (percent)	3.64%		3.6794%
Unsecured Term Loan \| Unsecured Debt
Line of Credit Facility [Line Items]
Debt instrument, face amount				$ 400,000,000
Unsecured term loans	$ 352,000,000		$ 352,000,000
Issuance costs, net of accumulated amortization	$ (2,804,000)		(3,145,000)
Unsecured Term Loan \| Unsecured Debt \| Interest Rate Swap One \| One-month LIBOR
Line of Credit Facility [Line Items]
Variable rate	1.20%
Unsecured Term Loan \| Line of Credit
Line of Credit Facility [Line Items]
Total debt, net	$ 349,196,000		$ 348,855,000