Unsecured Lines of Credit - Summary of Unsecured Lines of Credit (Details) - USD ($)		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Lines of credit		$ 50,000,000	$ 50,000,000
Bankers Trust Operating [Member]
Line Of Credit Facility [Line Items]
Lines of credit		0
Unsecured Lines of Credit [Member]
Line Of Credit Facility [Line Items]
Lines of credit		50,000,000	50,000,000
Line of credit facility maximum borrowing capacity		60,000,000	60,000,000
Unsecured Lines of Credit [Member] | 4.69% 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		4.69%
Unsecured Lines of Credit [Member] | 4.93% 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		4.93%
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%

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