Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Valuation allowance	$ 0		$ 0	$ 0	$ 0
U.S. federal income tax statutory rate			35.00%
Revaluation of net deferred income tax assets	$ 12,600,000		$ 12,620,000
Uncertain tax positions			$ 0	$ 0	$ 0
Scenario, Forecast [Member]
Income Tax Disclosure [Line Items]
U.S. federal income tax statutory rate		21.00%