Financing Activity - Credit Agreements (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) agreement quarter occasion	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Outstanding borrowings	$ 547,478,000	$ 547,289,000
Outstanding line of credit	$ 162,000,000	$ 65,000,000
Debt instrument, covenant compliance	As of March 31, 2019, the Borrower was in compliance with all financial covenants in the Credit Agreements.
Unsecured Debt
Debt Instrument [Line Items]
Debt issuance costs, line of credit arrangements	$ (2,500,000)
Credit Agreements
Debt Instrument [Line Items]
Number of credit agreements | agreement	2
Credit Agreements | 2018 Revolving Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 400,000,000
Outstanding line of credit	162,000,000
Remaining borrowing capacity	$ 196,800,000
Facility fee percentage	0.30%
Credit Agreements | 2018 Term Loan Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 300,000,000
Credit Agreements | 2014 Seven Year Term Loan
Debt Instrument [Line Items]
Maximum borrowing capacity	250,000,000
Credit Agreements | Term Loans
Debt Instrument [Line Items]
Outstanding borrowings	$ 550,000,000
Property With Average Sales Of More Than $500 Per Square Foot | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, capitalization rate	6.50%
Property Other Than With Average Sales Of $500 Per Square Foot | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, capitalization rate	7.50%
Covenant Term One | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, Minimum Tangible Net Worth	$ 1,463,200,000
Covenant compliance, percent of net proceeds of all equity issuances	75.00%
Covenant Term Two | Credit Agreements
Debt Instrument [Line Items]
Total liabilities to gross asset value, ratio	0.60
Covenant compliance, number of consecutive quarters total liabilities to gross asset value ratio exceeding threshold | quarter	2
Covenant compliance, number of occasions total liabilities to gross asset value ratio exceeding threshold | occasion	2
Covenant Term Three | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, ratio of adjusted EBITDA to fixed charges, minimum	1.50
Covenant Term Four, Through June 30, 2020 | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, minimum unencumbered debt yield	0.11
Covenant Term Four, After June 30, 2020 Through June 30, 2021 | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, minimum unencumbered debt yield	0.1125
Covenant Term Four, After June 30, 2021 | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, minimum unencumbered debt yield	0.115
Covenant Term Five | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, minimum unencumbered NOI to unsecured interest expense, ratio	1.75
Covenant Term Six | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, secured indebtedness to gross asset value, maximum ratio	0.60
Covenant Term Seven | Credit Agreements
Debt Instrument [Line Items]
Covenant compliance, maximum distribution percent of funds from operations	95.00%
Covenant compliance, maximum distribution percent of REIT taxable income	110.00%
Maximum | Covenant Term Two | Credit Agreements
Debt Instrument [Line Items]
Total liabilities to gross asset value, ratio	0.625