CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings
Balance at beginning of period at Dec. 31, 2017	$ 245,583		$ 209,603	$ 35,980
Balance at beginning of period (in shares) at Dec. 31, 2017		41,641
Exercise of common stock options	1,212		1,212
Exercise of common stock options (in shares)		415
Stock-based compensation	7,427		7,427
Restricted stock award grants, net of forfeitures and shares withheld for taxes	(361)		(361)
Restricted stock award grants, net of forfeitures and shares withheld for taxes (in shares)		13
Net income	9,070			9,070
Balance at end of period at Mar. 31, 2018	262,931		217,881	45,050
Balance at end of period (in shares) at Mar. 31, 2018		42,069
Balance at beginning of period at Dec. 31, 2017	245,583		209,603	35,980
Balance at beginning of period (in shares) at Dec. 31, 2017		41,641
Net income	48,705
Balance at end of period at Sep. 30, 2018	337,014		252,329	84,685
Balance at end of period (in shares) at Sep. 30, 2018		43,185
Balance at beginning of period at Mar. 31, 2018	262,931		217,881	45,050
Balance at beginning of period (in shares) at Mar. 31, 2018		42,069
Exercise of common stock options	1,997		1,997
Exercise of common stock options (in shares)		365
Stock-based compensation	9,872		9,872
Issuance of common stock under employee stock purchase plan	7,014		7,014
Issuance of common stock under employee stock purchase plan (in shares)		392
Restricted stock award grants, net of forfeitures and shares withheld for taxes	(183)		(183)
Restricted stock award grants, net of forfeitures and shares withheld for taxes (in shares)		28
Net income	19,343			19,343
Balance at end of period at Jun. 30, 2018	300,974		236,581	64,393
Balance at end of period (in shares) at Jun. 30, 2018		42,854
Exercise of common stock options	3,956		3,956
Exercise of common stock options (in shares)		330
Stock-based compensation	11,851		11,851
Restricted stock award grants, net of forfeitures and shares withheld for taxes	(59)		(59)
Restricted stock award grants, net of forfeitures and shares withheld for taxes (in shares)		1
Net income	20,292			20,292
Balance at end of period at Sep. 30, 2018	337,014		252,329	84,685
Balance at end of period (in shares) at Sep. 30, 2018		43,185
Balance at beginning of period at Dec. 31, 2018	394,567		270,447	124,120
Balance at beginning of period (in shares) at Dec. 31, 2018		43,864
Exercise of common stock options	9,502		9,502
Exercise of common stock options (in shares)		592
Stock-based compensation	16,324		16,324
Restricted stock award grants, net of forfeitures and shares withheld for taxes	(1,146)		(1,146)
Restricted stock award grants, net of forfeitures and shares withheld for taxes (in shares)		23
Net income	10,151			10,151
Balance at end of period at Mar. 31, 2019	429,398		295,127	134,271
Balance at end of period (in shares) at Mar. 31, 2019		44,479
Balance at beginning of period at Dec. 31, 2018	394,567		$ 270,447	$ 124,120
Balance at beginning of period (in shares) at Dec. 31, 2018		43,864
Net income	57,373
Balance at end of period at Sep. 30, 2019	$ 535,392
Balance at end of period (in shares) at Sep. 30, 2019	535,392	45,121	353,899	181,493
Balance at beginning of period at Mar. 31, 2019	$ 429,398		$ 295,127	$ 134,271
Balance at beginning of period (in shares) at Mar. 31, 2019		44,479
Exercise of common stock options	7,473		7,473
Exercise of common stock options (in shares)		284
Stock-based compensation	21,390		21,390
Issuance of common stock under employee stock purchase plan	8,648		8,648
Issuance of common stock under employee stock purchase plan (in shares)		153
Restricted stock award grants, net of forfeitures and shares withheld for taxes	(2,488)		(2,488)
Restricted stock award grants, net of forfeitures and shares withheld for taxes (in shares)		7
Net income	27,800			27,800
Balance at end of period at Jun. 30, 2019	492,221		330,150	162,071
Balance at end of period (in shares) at Jun. 30, 2019		44,923
Exercise of common stock options	4,936		4,936
Exercise of common stock options (in shares)		196
Stock-based compensation	19,923		19,923
Restricted stock award grants, net of forfeitures and shares withheld for taxes	(1,110)		$ (1,110)
Restricted stock award grants, net of forfeitures and shares withheld for taxes (in shares)		2
Net income	19,422			$ 19,422
Balance at end of period at Sep. 30, 2019	$ 535,392
Balance at end of period (in shares) at Sep. 30, 2019	535,392	45,121	353,899	181,493