Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common Class A [Member] Common Stock [Member]	Common Class A [Member] Additional Paid-in Capital [Member]	Common Class A [Member] Treasury Stock [Member]	Common Class A [Member] Retained Earnings [Member]	Common Class A [Member] AOCI Attributable to Parent [Member]	Common Class A [Member]	Common Class B [Member] Common Stock [Member]	Common Class B [Member] Additional Paid-in Capital [Member]	Common Class B [Member] Treasury Stock [Member]	Common Class B [Member] Retained Earnings [Member]	Common Class B [Member] AOCI Attributable to Parent [Member]	Common Class B [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2015	30,304,000						10,093,000
Balance at Dec. 31, 2015													$ 430	$ 288,294	$ (43,368)	$ 599,846	$ (305)	$ 844,897
Stock options exercised and stock awards (in shares)	494,000
Stock options exercised and stock awards													5	6,152				6,157
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan														12,875				12,875
Vesting of restricted share awards (in shares)	6,000						186,000
Vesting of restricted share awards													2	(725)				(723)
Issuance of common stock under employee stock purchase plan (in shares)	137,000
Issuance of common stock under employee stock purchase plan													1	2,531				2,532
Common stock repurchases (in shares)	(934,000)						(1,034,000)
Common stock repurchases															(43,514)			(43,514)
Other comprehensive income																	19	19
Net income																40,582		40,582
Balance (in shares) at Dec. 31, 2016	30,007,000						9,245,000
Balance at Dec. 31, 2016													438	309,127	(86,882)	640,428	(286)	862,825
Stock options exercised and stock awards (in shares)	1,219,000
Stock options exercised and stock awards													12	22,355				22,367
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan														15,606				15,606
Vesting of restricted share awards (in shares)	7,000						199,000
Vesting of restricted share awards													3	(1,518)				(1,515)
Issuance of common stock under employee stock purchase plan (in shares)	113,000
Issuance of common stock under employee stock purchase plan													1	2,474				2,475
Common stock repurchases (in shares)							(975,000)
Common stock repurchases															(33,800)			(33,800)
Other comprehensive income																	286	286
Net income																172,129		172,129
Balance (in shares) at Dec. 31, 2017	31,345,000						8,469,000
Balance at Dec. 31, 2017													454	348,044	(120,682)	812,557		$ 1,040,373
Stock options exercised and stock awards (in shares)	138,000																	126,027
Stock options exercised and stock awards													1	2,742				$ 2,743
Stock-based compensation related to stock options, restricted shares and employee stock purchase plan														18,059				18,059
Vesting of restricted share awards (in shares)							227,000
Vesting of restricted share awards													2	(1,749)				(1,747)
Issuance of common stock under employee stock purchase plan (in shares)	84,000
Issuance of common stock under employee stock purchase plan													1	2,929				2,930
Common stock repurchases (in shares)	(2,857,000)						(406,000)
Common stock repurchases															(125,160)			(125,160)
Other comprehensive income
Net income																139,062		139,062
Cash dividends declared on common stock				$ (7,324)		$ (7,324)				$ (2,008)		$ (2,008)
Balance (in shares) at Dec. 31, 2018	28,710,000						8,290,000
Balance at Dec. 31, 2018													$ 458	$ 370,025	$ (245,842)	$ 942,287		$ 1,066,928