Income Taxes - Reconciliation of Financial Statement Net Income to Taxable Income (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Net income per financial statements	$ 178,296	$ 178,597	$ 177,911	$ 142,722	$ 174,494	$ 176,260	$ 173,203	$ 134,918	$ 677,526	$ 658,875	$ 634,162
Excess book depreciation									17,328	17,328	17,328
Taxable income									$ 694,854	$ 676,203	$ 651,490