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	139,646	139,646
Other comprehensive income, net of tax	0	0	0	(16,237)	0	(16,237)
Repurchase of Class A common stock (Note 6)	0	0	(48,222)	0	0	(48,222)
Exercise of employee stock options/vesting of stock awards	0	1,772	7,223	0	0	8,995
Stock-based compensation	0	20,311	0	0	0	20,311
Business Acquisitions	0	80,064	19,794	0	0	99,858
Equity component of convertible note settlement (net)	0	(23,262)	19,887	0	0	(3,375)
Cash dividends	0	0	0	0	(60,396)	(60,396)
Balance at end of period at Sep. 30, 2018	91	545,234	(1,306,012)	(82,555)	1,675,376	832,134
Balance at beginning of period at Jun. 30, 2018	91	532,996	(1,296,628)	(75,460)	1,642,535	803,534
Stockholders' Equity [Roll Forward]
Net income	0	0	0	0	53,146	53,146
Other comprehensive income, net of tax	0	0	0	(7,095)	0	(7,095)
Repurchase of Class A common stock (Note 6)	0	0	(12,399)	0	0	(12,399)
Exercise of employee stock options/vesting of stock awards	0	1,714	1,330	0	0	3,044
Stock-based compensation	0	7,044	0	0	0	7,044
Business Acquisitions	0	3,480	1,685	0	0	5,165
Cash dividends	0	0	0	0	(20,305)	(20,305)
Balance at end of period at Sep. 30, 2018	91	545,234	(1,306,012)	(82,555)	1,675,376	832,134
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	133,441	133,441
Other comprehensive income, net of tax	0	0	0	(15,027)	0	(15,027)
Repurchase of Class A common stock (Note 6)	0	0	(825)	0	0	(825)
Exercise of employee stock options/vesting of stock awards	0	(4,930)	2,604	0	0	(2,326)
Stock-based compensation	0	9,649	0	0	0	9,649
Cash dividends	0	0	0	0	(61,636)	(61,636)
Balance at end of period at Sep. 30, 2019	91	557,283	(1,324,826)	(94,961)	1,708,213	845,800
Balance at beginning of period at Jun. 30, 2019	91	556,294	(1,324,834)	(81,276)	1,684,675	834,950
Stockholders' Equity [Roll Forward]
Net income	0	0	0	0	44,090	44,090
Other comprehensive income, net of tax	0	0	0	(13,685)	0	(13,685)
Repurchase of Class A common stock (Note 6)	0	0	0	0	0	0
Exercise of employee stock options/vesting of stock awards	0	(26)	8	0	0	(18)
Stock-based compensation	0	1,015	0	0	0	1,015
Cash dividends	0	0	0	0	(20,552)	(20,552)
Balance at end of period at Sep. 30, 2019	$ 91	$ 557,283	$ (1,324,826)	$ (94,961)	$ 1,708,213	$ 845,800