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Income taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. corporate statutory income tax rate	21.00%	34.00%
Effective income tax rate, percentage	3.00%	0.00%
Unrecognized tax benefits	$ 1,480,000		$ 1,480,000
Accrued interest and penalties to uncertain tax positions	$ 0

X

- Definition

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

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of unrecognized tax benefits.

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Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

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