Debt - Mortgages payable (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total principal	$ 60,926	$ 88,270
Debt [Member]
Debt Instrument [Line Items]
Total principal	60,900
Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Total principal	61,519	89,095
Unamortized debt issuance costs	(593)	(825)
Total	60,926	88,270
Unsecured Debt [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs	(2,124)	(1,133)
Total	258,543	159,304
October 2019 [Member] \| Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Total principal	21,500	21,500
January 2020 [Member] \| Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Total principal	1,922	3,573
February 2020 [Member] \| Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Total principal	2,872	2,963
January 1, 2023 [Member] \| Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Total principal	23,640	23,640
September 2023 [Member] \| Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Total principal	4,959	5,131
July 2026 [Member] \| Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Total principal	6,626	7,288
April4 2018 [Member] \| Mortgage Notes Payable [Member]
Debt Instrument [Line Items]
Total principal	$ 0	$ 25,000

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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