Income Taxes (Unaudited) - Reconciliation of Financial Statement Net Income to Taxable Income (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation Of Income Taxes [Line Items]
Net income per financial statements	$ 87,715	$ 89,307	$ 86,816	$ 56,709	$ 73,407	$ 85,818	$ 85,424	$ 56,431	$ 320,547	$ 301,080	$ 313,209
Excess (tax) book depreciation									(3,501)	(3,501)	14,915
Taxable income									$ 317,046	$ 297,579	$ 328,124