Stockholders' Equity - Summary of Purchase and Sale of Common Stock (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Shares of common stock purchased	8,220	11,530	56,325
Aggregate purchase price of shares purchased	$ 390,542	$ 291,349	$ 2,226,675
Shares of common stock sold	4,717	10,694	83,961
Aggregate sale price of shares sold	$ 271,417	$ 322,928	$ 3,380,448