Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock Class A common stock	Common Stock Class B common stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Retained Earnings/(Accumulated Deficit)
Beginning balance (in shares) at Dec. 31, 2018		15,789,000	18,109,000
Beginning balance at Dec. 31, 2018	$ 91,846	$ 2	$ 2	$ 157,898	$ (178)	$ (21,562)	$ (44,316)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		14,000
Exercise of stock options	90			90
Stock-based compensation	1,831			1,831
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		58,000
Vesting of restricted stock units, net of shares withheld for taxes	(2,572)			(2,572)
Vesting of early exercised shares	6			6
Conversion of Class B stock to Class A stock (in shares)		38,000	(38,000)
Conversion of Class B stock to Class A stock	0
Other comprehensive income (loss)	129				129
Net income	3,725						3,725
Ending balance (in shares) at Mar. 31, 2019		15,899,000	18,071,000
Ending balance at Mar. 31, 2019	95,055	$ 2	$ 2	157,253	(49)	(21,562)	(40,591)
Beginning balance (in shares) at Dec. 31, 2018		15,789,000	18,109,000
Beginning balance at Dec. 31, 2018	91,846	$ 2	$ 2	157,898	(178)	(21,562)	(44,316)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	31,854
Ending balance (in shares) at Sep. 30, 2019		16,373,000	17,685,000
Ending balance at Sep. 30, 2019	125,417	$ 2	$ 2	159,399	38	(21,562)	(12,462)
Beginning balance (in shares) at Mar. 31, 2019		15,899,000	18,071,000
Beginning balance at Mar. 31, 2019	95,055	$ 2	$ 2	157,253	(49)	(21,562)	(40,591)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		23,000
Exercise of stock options	109			109
Stock-based compensation	2,080			2,080
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		42,000
Vesting of restricted stock units, net of shares withheld for taxes	(2,247)			(2,247)
Conversion of Class B stock to Class A stock (in shares)		119,000	(119,000)
Conversion of Class B stock to Class A stock	0
Issuance of restricted stock awards (in shares)		3,000
Issuance of restricted stock awards	0
Other comprehensive income (loss)	67				67
Net income	23,151						23,151
Ending balance (in shares) at Jun. 30, 2019		16,086,000	17,952,000
Ending balance at Jun. 30, 2019	118,215	$ 2	$ 2	157,195	18	(21,562)	(17,440)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		9,000
Exercise of stock options	60			60
Stock-based compensation	2,678			2,678
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		11,000
Vesting of restricted stock units, net of shares withheld for taxes	(534)			(534)
Conversion of Class B stock to Class A stock (in shares)		267,000	(267,000)
Conversion of Class B stock to Class A stock	0
Issuance of restricted stock awards	0
Other comprehensive income (loss)	20				20
Net income	4,978						4,978
Ending balance (in shares) at Sep. 30, 2019		16,373,000	17,685,000
Ending balance at Sep. 30, 2019	125,417	$ 2	$ 2	159,399	38	(21,562)	(12,462)
Beginning balance (in shares) at Dec. 31, 2019		16,552,000	17,594,000
Beginning balance at Dec. 31, 2019	131,950	$ 2	$ 2	161,509	33	(21,562)	(8,034)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		17,000
Exercise of stock options	97			97
Stock-based compensation	1,365			1,365
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		91,000
Vesting of restricted stock units, net of shares withheld for taxes	(6,458)			(6,458)
Conversion of Class B stock to Class A stock (in shares)		58,000	(58,000)
Conversion of Class B stock to Class A stock	0
Other comprehensive income (loss)	$ 132				132
Repurchase of common stock (in shares)	(48,002)	(48,000)
Repurchase of common stock	$ (4,194)					(4,194)
Net income	1,983						1,983
Ending balance (in shares) at Mar. 31, 2020		16,670,000	17,536,000
Ending balance at Mar. 31, 2020	124,875	$ 2	$ 2	156,513	165	(25,756)	(6,051)
Beginning balance (in shares) at Dec. 31, 2019		16,552,000	17,594,000
Beginning balance at Dec. 31, 2019	$ 131,950	$ 2	$ 2	161,509	33	(21,562)	(8,034)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)	59,000
Net income	$ 158,916
Ending balance (in shares) at Sep. 30, 2020		18,031,000	16,282,000
Ending balance at Sep. 30, 2020	284,516	$ 2	$ 2	159,296	90	(25,756)	150,882
Beginning balance (in shares) at Mar. 31, 2020		16,670,000	17,536,000
Beginning balance at Mar. 31, 2020	124,875	$ 2	$ 2	156,513	165	(25,756)	(6,051)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		32,000
Exercise of stock options	232			232
Stock-based compensation	3,406			3,406
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		50,000
Vesting of restricted stock units, net of shares withheld for taxes	(3,232)			(3,232)
Conversion of Class B stock to Class A stock (in shares)		197,000	(197,000)
Conversion of Class B stock to Class A stock	0
Issuance of restricted stock awards (in shares)		3,000
Issuance of restricted stock awards	0
Other comprehensive income (loss)	(34)				(34)
Net income	19,257						19,257
Ending balance (in shares) at Jun. 30, 2020		16,952,000	17,339,000
Ending balance at Jun. 30, 2020	144,504	$ 2	$ 2	156,919	131	(25,756)	13,206
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		10,000
Exercise of stock options	74			74
Stock-based compensation	3,572			3,572
Vesting of restricted stock units, net of shares withheld for taxes (in shares)		12,000
Vesting of restricted stock units, net of shares withheld for taxes	(1,269)			(1,269)
Conversion of Class B stock to Class A stock (in shares)		1,057,000	(1,057,000)
Conversion of Class B stock to Class A stock	0
Other comprehensive income (loss)	(41)				(41)
Net income	137,676						137,676
Ending balance (in shares) at Sep. 30, 2020		18,031,000	16,282,000
Ending balance at Sep. 30, 2020	$ 284,516	$ 2	$ 2	$ 159,296	$ 90	$ (25,756)	$ 150,882