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Income Taxes - Narrative (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ 5,726,807	$ 6,496,980	$ 14,446,092	$ 19,018,708
Effective tax rate	25.90%	33.10%	24.60%	32.60%
Federal statutory income tax rate, percent	21.00%	35.00%	21.00%	35.00%
Effective income tax rate reconciliation, share-based compensation, excess tax benefit (tax shortfall), Amount	$ 200,000	$ 400,000	$ 700,000	$ 1,700,000
Valuation allowance	0		0		$ 0
Liability for uncertain tax positions	$ 800,000		$ 800,000		$ 0

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- Definition

Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit (Tax Shortfall), Amount

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount recognized for uncertainty in income taxes classified as current.

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