CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2015	$ 466,103	$ 197,826	$ 273,764	$ (5,487)
Beginning balance (in shares) at Dec. 31, 2015		44,267
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,121		20,121
Other comprehensive income (loss)	3,605			3,605
Excess tax benefits from stock-based compensation	669	$ 669
Stock-based compensation expense	2,506	2,506
Options exercised	4,923	$ 4,923
Options exercised (in shares)		362
Issuance of common stock under Employee Stock Purchase Plans	694	$ 694
Issuance of common stock under Employee Stock Purchase Plans (in shares)		34
Shares surrendered in exchange for payment of payroll tax liabilities	(86)	$ (86)
Shares surrendered in exchange for payment of payroll tax liabilities (in shares)		(4)
Shares surrendered in exchange for exercise of stock options	(346)	$ (346)
Shares surrendered in exchange for the exercise of stock options (in shares)		(14)
Ending balance at Dec. 31, 2016	498,189	$ 206,186	293,885	(1,882)
Ending balance (in shares) at Dec. 31, 2016		44,645
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	27,523		27,523
Other comprehensive income (loss)	3,416			3,416
Stock-based compensation expense	4,075	$ 4,075
Options exercised	5,689	$ 5,689
Options exercised (in shares)		404
Issuance of common stock under Employee Stock Purchase Plans	836	$ 836
Issuance of common stock under Employee Stock Purchase Plans (in shares)		24
Issuance of common stock, net of offering costs	136,606	$ 136,606
Issuance of common stock, net of offering costs (in shares)		5,175
Shares surrendered in exchange for exercise of stock options	0
Shares surrendered in exchange for the exercise of stock options (in shares)		0
Ending balance at Dec. 31, 2017	$ 676,334	$ 353,392	321,408	1,534
Ending balance (in shares) at Dec. 31, 2017	50,248	50,248
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 42,017		42,017
Other comprehensive income (loss)	(3,567)			(3,567)
Stock-based compensation expense	6,117	$ 6,117
Options exercised	$ 10,634	$ 10,634
Options exercised (in shares)	690	690
Issuance of common stock under Employee Stock Purchase Plans	$ 1,087	$ 1,087
Issuance of common stock under Employee Stock Purchase Plans (in shares)		22
Issuance of common stock, net of offering costs	205,030	$ 205,030
Issuance of common stock, net of offering costs (in shares)		4,025
Shares surrendered in exchange for payment of payroll tax liabilities	(2,616)	$ (2,616)
Shares surrendered in exchange for payment of payroll tax liabilities (in shares)		(49)
Shares surrendered in exchange for exercise of stock options	(2,261)	$ (2,261)
Shares surrendered in exchange for the exercise of stock options (in shares)		(43)
Ending balance at Dec. 31, 2018	$ 932,775	$ 571,383	$ 363,425	$ (2,033)
Ending balance (in shares) at Dec. 31, 2018	54,893	54,893