Federal Income Taxes - Schedule of Characterized Distributions Paid per Common Share (Details) - $ / shares	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Characterization Of Distributions [Line Items]
Amount of distributions paid per common share	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.72	$ 0.72	$ 0.72
Percentage of distributions paid per common share													100.00%	100.00%	100.00%
Ordinary Income [Member]
Characterization Of Distributions [Line Items]
Amount of distributions paid per common share													$ 0.00000	$ 0.09549	$ 0.00000
Percentage of distributions paid per common share													0.00%	13.26%	0.00%
Capital Gains [Member]
Characterization Of Distributions [Line Items]
Amount of distributions paid per common share													$ 0.00000	$ 0.01425	$ 0.00000
Percentage of distributions paid per common share													0.00%	1.98%	0.00%
Return Of Capital [Member]
Characterization Of Distributions [Line Items]
Amount of distributions paid per common share													$ 0.72000	$ 0.61026	$ 0.72000
Percentage of distributions paid per common share													100.00%	84.76%	100.00%