Consolidated Statement of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock and paid-in capital	Retained earnings	Treasury stock	Shares issued
Balance, beginning of period at Dec. 31, 2018		$ 426,737	$ 422,235	$ (135,576)
Balance, beginning of period (in shares) at Dec. 31, 2018				13,663,194	98,924,501
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		39,175
Stock-based compensation expense		18,715
Vesting of restricted stock units, net of shares withheld for employee tax		(6,970)
Net income	$ 69,878		69,878
Exercise of stock options (in shares)	2,427,156				2,427,156
Vesting of restricted stock units, net of shares withheld for employee tax (in shares)					405,022
Balance, end of period at Sep. 30, 2019	$ 834,194	477,657	492,113	$ (135,576)
Balance, end of period (in shares) at Sep. 30, 2019				13,663,194	101,756,679
Balance, beginning of period at Jun. 30, 2019		462,569	470,977	$ (135,576)
Balance, beginning of period (in shares) at Jun. 30, 2019				13,663,194	101,014,406
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		12,177
Stock-based compensation expense		2,934
Vesting of restricted stock units, net of shares withheld for employee tax		(23)
Net income	$ 21,136		21,136
Exercise of stock options (in shares)	740,748				740,748
Vesting of restricted stock units, net of shares withheld for employee tax (in shares)					1,525
Balance, end of period at Sep. 30, 2019	$ 834,194	477,657	492,113	$ (135,576)
Balance, end of period (in shares) at Sep. 30, 2019				13,663,194	101,756,679
Balance, beginning of period at Dec. 31, 2019	854,865	480,982	509,459	$ (135,576)
Balance, beginning of period (in shares) at Dec. 31, 2019				13,663,194	101,766,468
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		2,145
Stock-based compensation expense		21,083
Vesting of restricted stock units, net of shares withheld for employee tax		(3,484)
Net income	$ 37,330		37,330
Exercise of stock options (in shares)	119,991				119,991
Vesting of restricted stock units, net of shares withheld for employee tax (in shares)					320,561
Balance, end of period at Sep. 30, 2020	$ 911,939	500,726	546,789	$ (135,576)
Balance, end of period (in shares) at Sep. 30, 2020				13,663,194	102,207,020
Balance, beginning of period at Jun. 30, 2020		497,091	528,754	$ (135,576)
Balance, beginning of period (in shares) at Jun. 30, 2020				13,663,194	102,173,154
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options		526
Stock-based compensation expense		3,133
Vesting of restricted stock units, net of shares withheld for employee tax		(24)
Net income	$ 18,035		18,035
Exercise of stock options (in shares)	28,402				28,402
Vesting of restricted stock units, net of shares withheld for employee tax (in shares)					5,464
Balance, end of period at Sep. 30, 2020	$ 911,939	$ 500,726	$ 546,789	$ (135,576)
Balance, end of period (in shares) at Sep. 30, 2020				13,663,194	102,207,020