Earnings Per Share - Summary of Reconciliation of Numerators and Denominators Used in Basic and Diluted Per Share Computations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Class A Common Stock [Member]
Numerator:
Allocation of net income (loss)	$ 9,768	$ 4,269	$ 31,495	$ 27,065
Denominator:
Weighted-average shares outstanding	23,767	23,015	23,494	22,933
Basic earnings (loss) per share	$ 0.41	$ 0.19	$ 1.34	$ 1.18
Numerator:
Allocation of net income (loss)	$ 9,768	$ 4,269	$ 31,495	$ 27,065
Denominator:
Number of shares used in basic computation	23,767	23,015	23,494	22,933
Weighted-average shares effect of dilutive securities:
Director and employee stock options	170	277	185	183
Number of shares used in diluted computation	23,937	23,292	23,679	23,116
Diluted earnings (loss) per share	$ 0.41	$ 0.18	$ 1.33	$ 1.17
Class B Common Stock [Member]
Numerator:
Allocation of net income (loss)	$ 2,069	$ 917	$ 6,752	$ 5,933
Denominator:
Weighted-average shares outstanding	5,577	5,577	5,577	5,577
Basic earnings (loss) per share	$ 0.37	$ 0.16	$ 1.21	$ 1.06
Numerator:
Allocation of net income (loss)	$ 2,069	$ 917	$ 6,752	$ 5,933
Denominator:
Number of shares used in basic computation	5,577	5,577	5,577	5,577
Weighted-average shares effect of dilutive securities:
Number of shares used in diluted computation	5,577	5,577	5,577	5,577
Diluted earnings (loss) per share	$ 0.37	$ 0.16	$ 1.21	$ 1.06