Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Fair Value Measurements at Reporting Date

(in thousands)	As of December 31, 2018
	Carrying Value		Fair Value
			Level 1		Level 2		Level 3
Wells Fargo revolving credit facility	$	54,613	$	—	$	54,613	$	—
Unsecured Senior Notes	55,000		—		—		52,700

(in thousands)	As of December 31, 2017
	Carrying Value		Fair Value
			Level 1		Level 2		Level 3
Wells Fargo revolving credit facility	$	37,055	$	—	$	37,055	$	—
Unsecured Senior Notes	55,000		—		—		50,000

(in thousands)	As of December 31, 2018
	Carrying Value		Fair Value
			Level 1		Level 2		Level 3
Weichai Warrant liability	$	35,100	$	—	$	—	$	35,100

(in thousands)	As of December 31, 2017
	Carrying Value		Fair Value
			Level 1		Level 2		Level 3
Weichai Warrant liability	$	24,700	$	—	$	—	$	24,700

Summary of Change in the Estimated Fair Value of Private Placement Warrants

The following table summarizes changes in the estimated fair value of the Company’s warrant liability:

(in thousands)	As of December 31,
	2018		2017
Balance at beginning of year	$	24,700	$	—
Issuance of warrants	—		20,700
Increase in value*	10,400		4,000
Balance at end of year	$	35,100	$	24,700

* Loss related to the change in fair value of the warrant liability for each year is presented in Loss from change in fair value of warrants in the Company’s Consolidated Statements of Operations.

Schedule of Inputs of the Black-Scholes Option Pricing Model

The inputs of the Black-Scholes option pricing model were as follows:

Assumptions	As of December 31,
	2018			2017
Market value of the Common Stock	$	9.25		$	7.50
Exercise price	varies			varies
Risk-free interest rate	2.6		%	1.8		%
Estimated price volatility	55.0		%	95.0		%
Contractual term	0.5 years			1.0 year
Dividend yield	—			—