Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) a	Dec. 31, 2017 USD ($)	Mar. 31, 2019 USD ($)
Significant Accounting Policies [Line Items]
Severance costs	$ 924,000	$ 287,000
Allowance for doubtful accounts	1,279,000	1,602,000
Other receivables	442,000	546,000
Restricted cash	10,565,000	10,216,000
Lower of cost or market inventory adjustment	0	0
Production cost	32,509,000	40,664,000
Start-up costs	0	0
Research and development expenses	$ 3,823,000	$ 4,417,000
Accounting Standards Update 2018-11 | Other Long Term Liabilities and Accrued Expenses | Subsequent Event
Significant Accounting Policies [Line Items]
Deferred rent			$ 8,300,000
Georgia and Alabama
Significant Accounting Policies [Line Items]
Area of land and leasehold interests | a	2,957
Marshfield, Wisconsin
Significant Accounting Policies [Line Items]
Area of land and leasehold interests | a	469
Maximum
Significant Accounting Policies [Line Items]
Receivables collection period	30 days
Maturity period for all highly liquid investments when purchased to be cash equivalents	3 months
Capitalized software estimated useful life	5 years
Maximum | Accounting Standards Update 2018-11 | Subsequent Event
Significant Accounting Policies [Line Items]
Company's ROU asset			60,000,000
Total lease liability			68,000,000
Minimum | Accounting Standards Update 2018-11 | Subsequent Event
Significant Accounting Policies [Line Items]
Company's ROU asset			54,000,000
Total lease liability			$ 62,000,000
Eufaula and Alabama
Significant Accounting Policies [Line Items]
Percentage of value of assets of project on construction in progress	82.00%
Projects completed percentage	75.00%