Financing Activity - Credit Agreements (Details)		9 Months Ended
	Sep. 30, 2019 USD ($) Agreement	Sep. 30, 2019 USD ($) Agreement qtr Occasion	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Outstanding line of credit	$ 213,000,000	$ 213,000,000	$ 65,000,000
Debt instrument, covenant compliance		As of September 30, 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	$ (2,000,000.0)	$ (2,000,000.0)
Credit Agreements
Debt Instrument [Line Items]
Number of credit agreements | Agreement	2	2
Credit Agreements | Covenant Term One
Debt Instrument [Line Items]
Covenant compliance, Minimum Tangible Net Worth	$ 1,463,200,000	$ 1,463,200,000
Covenant compliance, percent of net proceeds of all equity issuances		75.00%
Credit Agreements | Covenant Term Two
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 | qtr		2
Covenant compliance, number of occasions total liabilities to gross asset value ratio exceeding threshold | Occasion		2
Credit Agreements | Covenant Term Two | Maximum
Debt Instrument [Line Items]
Total liabilities to gross asset value, ratio		0.625
Credit Agreements | Covenant Term Three
Debt Instrument [Line Items]
Covenant compliance, ratio of adjusted EBITDA to fixed charges, minimum		1.50
Credit Agreements | Covenant Term Four, Through September 30, 2020
Debt Instrument [Line Items]
Covenant compliance, minimum unencumbered debt yield		0.110
Credit Agreements | Covenant Term Four, After September 30, 2020 Through September 30, 2021
Debt Instrument [Line Items]
Covenant compliance, minimum unencumbered debt yield		0.1125
Credit Agreements | Covenant Term Four, After September 30, 2021
Debt Instrument [Line Items]
Covenant compliance, minimum unencumbered debt yield		0.1150
Credit Agreements | Covenant Term Five
Debt Instrument [Line Items]
Covenant compliance, minimum unencumbered NOI to unsecured interest expense, ratio		1.75
Credit Agreements | Covenant Term Six
Debt Instrument [Line Items]
Covenant compliance, secured indebtedness to gross asset value, maximum ratio		0.60
Credit Agreements | Covenant Term Seven
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%
Credit Agreements | Property With Average Sales Of More Than $500 Per Square Foot
Debt Instrument [Line Items]
Covenant compliance, capitalization rate		6.50%
Credit Agreements | Property Other Than With Average Sales Of $500 Per Square Foot
Debt Instrument [Line Items]
Covenant compliance, capitalization rate		7.50%
Credit Agreements | 2018 Revolving Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	400,000,000	$ 400,000,000
Outstanding line of credit	213,000,000.0	213,000,000.0
Remaining borrowing capacity	$ 80,700,000	80,700,000
Facility fee percentage	0.55%
Credit Agreements | 2018 Term Loan Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 300,000,000	300,000,000
Credit Agreements | 2014 Seven Year Term Loan
Debt Instrument [Line Items]
Maximum borrowing capacity	250,000,000	250,000,000
Credit Agreements | Term Loans
Debt Instrument [Line Items]
Outstanding borrowings	$ 550,000,000.0	$ 550,000,000.0