Unsecured Lines of Credit - Summary of Unsecured Lines of Credit (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	May 31, 2017	Apr. 30, 2017	Mar. 31, 2016	Feb. 29, 2016
Line Of Credit Facility [Line Items]
Lines of credit		$ 50,000,000	$ 40,000,000
Unsecured Lines of Credit [Member]
Line Of Credit Facility [Line Items]
Lines of credit		50,000,000	40,000,000
Line of credit facility maximum borrowing capacity		60,000,000	$ 47,500,000
Unsecured Lines of Credit [Member] | Bankers Trust [Member]
Line Of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity		$ 50,000,000			$ 50,000,000
Debt instrument, maturity date		May 31, 2019	May 31, 2019
Unsecured Lines of Credit [Member] | Bankers Trust Operating [Member]
Line Of Credit Facility [Line Items]
Lines of credit		$ 0
Line of credit facility maximum borrowing capacity				$ 10,000,000	$ 7,500,000	$ 7,500,000	$ 5,000,000
Unsecured Lines of Credit [Member] | 4.38% Interest Bearing Line of Credit [Member] | Bankers Trust [Member]
Line Of Credit Facility [Line Items]
Lines of credit		50,000,000
Line of credit facility maximum borrowing capacity		$ 50,000,000
Debt instrument, maturity date		May 31, 2019
Variable / Fixed	[1]	Variable
Reset Frequency		Monthly
Line of credit facility, interest rate during period		4.38%
Unsecured Lines of Credit [Member] | 4.62% Interest Bearing Line of Credit [Member] | Bankers Trust Operating [Member]
Line Of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity		$ 10,000,000
Debt instrument, maturity date		May 31, 2019
Variable / Fixed	[1]	Variable
Reset Frequency		Monthly
Line of credit facility, interest rate during period		4.62%
Unsecured Lines of Credit [Member] | 3.13% Interest Bearing Line of Credit [Member] | Bankers Trust [Member]
Line Of Credit Facility [Line Items]
Lines of credit			$ 40,000,000
Line of credit facility maximum borrowing capacity			$ 40,000,000
Debt instrument, maturity date			May 31, 2018
Variable / Fixed	[1]		Variable
Reset Frequency			Monthly
Line of credit facility, interest rate during period			3.13%
Unsecured Lines of Credit [Member] | 3.88% Interest Bearing Line of Credit [Member] | Bankers Trust Operating [Member]
Line Of Credit Facility [Line Items]
Line of credit facility maximum borrowing capacity			$ 7,500,000
Debt instrument, maturity date			May 31, 2018
Variable / Fixed	[1]		Variable
Reset Frequency			Monthly
Line of credit facility, interest rate during period			3.88%

[1] The variable rate is indexed to LIBOR plus an applicable margin