CONSOLIDATED STATEMENTS OF STOCKHOLDERS EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock Cumulative Effect, Period of Adoption, as Adjusted	Common Stock	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period of Adoption, as Adjusted	Retained Earnings	Treasury Stock Cumulative Effect, Period of Adoption, as Adjusted	Treasury Stock	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, as Adjusted	Total
Balance at Dec. 31, 2019	$ 686,806	$ 686,806	$ (11,639)	$ 2,067,540	$ 2,079,179	$ (590,971)	$ (590,971)	$ (11,639)	$ 2,163,375	$ 2,175,014
Balance (in shares) at Dec. 31, 2019	81,743	81,743				(31,420)	(31,420)
Increase (Decrease) in Stockholders' Equity
Net income					29,988					29,988
Exercise of common stock options and vested employee stock awards		$ 38								38
Exercise of common stock options and vested employee stock awards (in shares)		287
Employee income tax paid on vested equity awards							$ (6,165)			(6,165)
Employee income tax paid on vested equity awards (in shares)							(108)
Sale of common stock under employee stock purchase plan		$ 1,494								1,494
Sale of common stock under employee stock purchase plan (in shares)		24
Stock based compensation expense		$ 1,727								1,727
Treasury stock purchases							$ (20,000)			(20,000)
Treasury stock purchases (in shares)							(386)
Cash dividends declared ($0.14 per share)					(7,019)					(7,019)
Balance at Mar. 31, 2020		$ 690,065			2,090,509		$ (617,136)			2,163,438
Balance (in shares) at Mar. 31, 2020		82,054					(31,914)
Balance at Dec. 31, 2020		$ 704,675			2,052,006		$ (617,136)			2,139,545
Balance (in shares) at Dec. 31, 2020		82,095					(31,914)
Increase (Decrease) in Stockholders' Equity
Net income					35,900					35,900
Exercise of common stock options and vested employee stock awards		$ 606								606
Exercise of common stock options and vested employee stock awards (in shares)		177
Employee income tax paid on vested equity awards							$ (1,573)			(1,573)
Employee income tax paid on vested equity awards (in shares)							(42)
Sale of common stock under employee stock purchase plan		$ 1,139								1,139
Sale of common stock under employee stock purchase plan (in shares)		30
Stock based compensation expense		$ 2,613								2,613
Warrants issued to U.S. Treasury		3,291								3,291
Balance at Mar. 31, 2021		$ 712,324			$ 2,087,906		$ (618,709)			$ 2,181,521
Balance (in shares) at Mar. 31, 2021		82,302					(31,956)