UNAUDITED CONDENSED CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS’ EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Shares at beginning of period (in shares) at Dec. 31, 2017		71,494				45,167
Balance at beginning of period at Dec. 31, 2017	$ 134,277	$ 715	$ 437,394	$ 100	$ 195,143	$ (499,075)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	9,175				9,175
Issuance for stock-based compensation and dividends, net of forfeitures (in shares)		63
Issuance for stock-based compensation and dividends, net of forfeitures	0	$ 1	166		(167)
Exercise of stock options (in shares)		5				1
Exercise of stock options	0		46			$ (46)
Stock-based compensation expense	2,260		2,260
Employee stock purchase plan (in shares)						(6)
Employee stock purchase plan	132		71			$ 61
Dividends	(2,973)				(2,973)
Change in fair value of interest swap, net of tax	517			517
Repurchases of common stock (in shares)						319
Repurchases of common stock	(8,715)					$ (8,715)
Shares at end of period (in shares) at Mar. 31, 2018		71,562				45,481
Balance at end of period at Mar. 31, 2018	134,494	$ 716	439,937	617	200,999	$ (507,775)
Shares at beginning of period (in shares) at Dec. 31, 2017		71,494				45,167
Balance at beginning of period at Dec. 31, 2017	134,277	$ 715	437,394	100	195,143	$ (499,075)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41,624
Employee stock purchase plan	405
Change in fair value of interest swap, net of tax	807
Shares at end of period (in shares) at Sep. 30, 2018		71,544				45,483
Balance at end of period at Sep. 30, 2018	163,947	$ 716	444,762	907	225,658	$ (508,096)
Shares at beginning of period (in shares) at Mar. 31, 2018		71,562				45,481
Balance at beginning of period at Mar. 31, 2018	134,494	$ 716	439,937	617	200,999	$ (507,775)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16,272				16,272
Issuance for stock-based compensation and dividends, net of forfeitures (in shares)		34
Issuance for stock-based compensation and dividends, net of forfeitures	0		158		(158)
Stock-based compensation expense	2,292		2,292
Employee stock purchase plan (in shares)						(5)
Employee stock purchase plan	129		73			$ 56
Dividends	(2,970)				(2,970)
Change in fair value of interest swap, net of tax	180			180
Repurchases of common stock (in shares)						3
Repurchases of common stock	(91)					$ (91)
Shares at end of period (in shares) at Jun. 30, 2018		71,596				45,479
Balance at end of period at Jun. 30, 2018	150,306	$ 716	442,460	797	214,143	$ (507,810)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16,177				16,177
Issuance for stock-based compensation and dividends, net of forfeitures (in shares)		(52)
Issuance for stock-based compensation and dividends, net of forfeitures	0		201		(201)
Stock-based compensation expense	2,006		2,006
Employee stock purchase plan (in shares)						(4)
Employee stock purchase plan	144		95			$ 49
Dividends	(4,461)				(4,461)
Change in fair value of interest swap, net of tax	110			110
Repurchases of common stock (in shares)						8
Repurchases of common stock	(335)					$ (335)
Shares at end of period (in shares) at Sep. 30, 2018		71,544				45,483
Balance at end of period at Sep. 30, 2018	163,947	$ 716	444,762	907	225,658	$ (508,096)
Shares at beginning of period (in shares) at Dec. 31, 2018		71,856				45,822
Balance at beginning of period at Dec. 31, 2018	168,331	$ 719	447,337	1,296	237,308	$ (518,329)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	26,855				26,855
Reclassification of stranded tax effects (Note A) | Accounting Standards Update 2018-02	0			168	(168)
Issuance for stock-based compensation and dividends, net of forfeitures (in shares)		4
Issuance for stock-based compensation and dividends, net of forfeitures	0		233		(233)
Stock-based compensation expense	2,620		2,620
Employee stock purchase plan (in shares)						(5)
Employee stock purchase plan	140		86			$ 54
Dividends	(4,406)				(4,406)
Change in fair value of interest rate swap, net of tax	(280)			(280)
Repurchases of common stock (in shares)						432
Repurchases of common stock	(14,688)					$ (14,688)
Shares at end of period (in shares) at Mar. 31, 2019		71,860				46,249
Balance at end of period at Mar. 31, 2019	178,572	$ 719	450,276	1,184	259,356	$ (532,963)
Shares at beginning of period (in shares) at Dec. 31, 2018		71,856				45,822
Balance at beginning of period at Dec. 31, 2018	168,331	$ 719	447,337	1,296	237,308	$ (518,329)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	116,654
Employee stock purchase plan	427
Change in fair value of interest rate swap, net of tax	(871)
Shares at end of period (in shares) at Sep. 30, 2019		71,874				48,458
Balance at end of period at Sep. 30, 2019	187,925	$ 719	456,802	593	340,436	$ (610,625)
Shares at beginning of period (in shares) at Mar. 31, 2019		71,860				46,249
Balance at beginning of period at Mar. 31, 2019	178,572	$ 719	450,276	1,184	259,356	$ (532,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	74,859				74,859
Issuance for stock-based compensation and dividends, net of forfeitures (in shares)		5
Issuance for stock-based compensation and dividends, net of forfeitures	0		177		(177)
Stock-based compensation expense	3,524		3,524
Employee stock purchase plan (in shares)						(4)
Employee stock purchase plan	143		94			$ 49
Dividends	(4,278)				(4,278)
Change in fair value of interest rate swap, net of tax	(478)			(478)
Repurchases of common stock (in shares)						1,048
Repurchases of common stock	(37,486)					$ (37,486)
Shares at end of period (in shares) at Jun. 30, 2019		71,865				47,293
Balance at end of period at Jun. 30, 2019	214,856	$ 719	454,071	706	329,760	$ (570,400)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	14,940				14,940
Issuance for stock-based compensation and dividends, net of forfeitures (in shares)		9
Issuance for stock-based compensation and dividends, net of forfeitures	0		221		(221)
Stock-based compensation expense	2,419		2,419
Employee stock purchase plan (in shares)						(4)
Employee stock purchase plan	144		91			$ 53
Dividends	(4,043)				(4,043)
Change in fair value of interest rate swap, net of tax	(113)			(113)
Repurchases of common stock (in shares)						1,169
Repurchases of common stock	(40,278)					$ (40,278)
Shares at end of period (in shares) at Sep. 30, 2019		71,874				48,458
Balance at end of period at Sep. 30, 2019	$ 187,925	$ 719	$ 456,802	$ 593	$ 340,436	$ (610,625)