Basic and Diluted Net Income (Loss) Per Common Share - Summary of Computation of Basic and Diluted Net Income (Loss) Available to Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Class A common stock
Basic net income (loss) per share:
Allocation of undistributed earnings (losses)	$ 5,002	$ (4,063)	$ 1,901	$ (8,806)
Number of shares used in per share computation (as converted)	4,234,000	4,167,000	4,217,000	4,167,000
Basic net income (loss) per share	$ 1.18	$ (0.98)	$ 0.45	$ (2.11)
Diluted net income (loss) per share:
Allocation of undistributed earnings (losses)	$ 5,002	$ (4,063)	$ 1,901	$ (8,806)
Number of shares used in per share computation (as converted)	4,234,000	4,167,000	4,217,000	4,167,000
Weighted-average impact of dilutive equity instruments	9,000
Number of shares used in per share computation (as converted)	4,243,000	4,167,000	4,217,000	4,167,000
Diluted net income (loss) per share	$ 1.18	$ (0.98)	$ 0.45	$ (2.11)
Common stock equivalents excluded from calculation of diluted net income (loss) per share as the effect would have been anti-dilutive:	380	394	380	394
Class B common stock
Basic net income (loss) per share:
Allocation of undistributed earnings (losses)	$ 2,765	$ (2,282)	$ 1,055	$ (4,945)
Number of shares used in per share computation (as converted)	2,340,000	2,340,000	2,340,000	2,340,000
Basic net income (loss) per share	$ 1.18	$ (0.98)	$ 0.45	$ (2.11)
Diluted net income (loss) per share:
Allocation of undistributed earnings (losses)	$ 2,765	$ (2,282)	$ 1,055	$ (4,945)
Number of shares used in per share computation (as converted)	2,340,000	2,340,000	2,340,000	2,340,000
Number of shares used in per share computation (as converted)	2,340,000	2,340,000	2,340,000	2,340,000
Diluted net income (loss) per share	$ 1.18	$ (0.98)	$ 0.45	$ (2.11)
Series C convertible preferred stock
Basic net income (loss) per share:
Allocation of undistributed earnings (losses)	$ 898	$ (741)	$ 342	$ (1,606)
Number of shares used in per share computation (as converted)	760,000	760,000	760,000	760,000
Basic net income (loss) per share	$ 1.18	$ (0.98)	$ 0.45	$ (2.11)
Diluted net income (loss) per share:
Allocation of undistributed earnings (losses)	$ 898	$ (741)	$ 342	$ (1,606)
Number of shares used in per share computation (as converted)	760,000	760,000	760,000	760,000
Number of shares used in per share computation (as converted)	760,000	760,000	760,000	760,000
Diluted net income (loss) per share	$ 1.18	$ (0.98)	$ 0.45	$ (2.11)