Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [Member] Class A [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Total
Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of ASC Topic | ASC Topic 606 [Member]	$ 0	$ 0	$ 0	$ 0	$ (13,042)	$ (13,042)
Balance at beginning of period at Dec. 31, 2017	91	466,349	(1,304,694)	(66,318)	1,609,168	704,596
Stockholders' Equity [Roll Forward]
Net income	0	0	0	0	86,500	86,500
Other comprehensive income, net of tax	0	0	0	(9,142)	0	(9,142)
Repurchase of Class A common stock (Note 6)	0	0	(35,823)	0	0	(35,823)
Exercise of employee stock options/vesting of stock awards	0	58	5,893	0	0	5,951
Stock-based compensation	0	13,267	0	0	0	13,267
Business Acquisitions (1.4 million shares)	0	76,584	18,109	0	0	94,693
Equity component of convertible note settlement (net)	0	(23,262)	19,887	0	0	(3,375)
Cash dividends	0	0	0	0	(40,091)	(40,091)
Balance at end of period at Jun. 30, 2018	91	532,996	(1,296,628)	(75,460)	1,642,535	803,534
Balance at beginning of period at Mar. 31, 2018	91	523,233	(1,280,586)	(55,474)	1,611,864	799,128
Stockholders' Equity [Roll Forward]
Net income	0	0	0	0	50,961	50,961
Other comprehensive income, net of tax	0	0	0	(19,986)	0	(19,986)
Repurchase of Class A common stock (Note 6)	0	0	(18,437)	0	0	(18,437)
Exercise of employee stock options/vesting of stock awards	0	3,257	2,395	0	0	5,652
Stock-based compensation	0	6,506	0	0	0	6,506
Cash dividends	0	0	0	0	(20,290)	(20,290)
Balance at end of period at Jun. 30, 2018	91	532,996	(1,296,628)	(75,460)	1,642,535	803,534
Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of ASC Topic | ASC Topic 842 [Member]	0	0	0	0	657	657
Balance at beginning of period at Dec. 31, 2018	91	552,564	(1,326,605)	(79,934)	1,635,751	781,867
Stockholders' Equity [Roll Forward]
Net income	0	0	0	0	89,351	89,351
Other comprehensive income, net of tax	0	0	0	(1,342)	0	(1,342)
Repurchase of Class A common stock (Note 6)	0	0	(825)	0	0	(825)
Exercise of employee stock options/vesting of stock awards	0	(4,904)	2,596	0	0	(2,308)
Stock-based compensation	0	8,634	0	0	0	8,634
Cash dividends	0	0	0	0	(41,084)	(41,084)
Balance at end of period at Jun. 30, 2019	91	556,294	(1,324,834)	(81,276)	1,684,675	834,950
Balance at beginning of period at Mar. 31, 2019	91	553,270	(1,325,251)	(75,794)	1,658,879	811,195
Stockholders' Equity [Roll Forward]
Net income	0	0	0	0	46,349	46,349
Other comprehensive income, net of tax	0	0	0	(5,482)	0	(5,482)
Repurchase of Class A common stock (Note 6)	0	0	0	0	0	0
Exercise of employee stock options/vesting of stock awards	0	(569)	417	0	0	(152)
Stock-based compensation	0	3,593	0	0	0	3,593
Cash dividends	0	0	0	0	(20,553)	(20,553)
Balance at end of period at Jun. 30, 2019	$ 91	$ 556,294	$ (1,324,834)	$ (81,276)	$ 1,684,675	$ 834,950