Loss Per Share - Calculation of Loss Per Share (Parenthetical) (Detail) - Series B Stock [Member] - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible preferred stock interest rate	7.00%	7.00%	7.00%	7.00%	7.00%
Suspended dividend payment			$ 3,500,000
Dividends declared	$ 0	$ 0	$ 0	$ 0

Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding.

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