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 Loss [Member]	Retained Earnings [Member]	Total
Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of ASU | ASC 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	35,539	35,539
Other comprehensive income, net of tax	0	0	0	10,844	0	10,844
Repurchase of Class A common stock (Note 6)	0	0	(17,386)	0	0	(17,386)
Exercise of employee stock options/vesting of stock awards	0	(3,199)	3,498	0	0	299
Stock-based compensation	0	6,761	0	0	0	6,761
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	(19,801)	(19,801)
Balance at end of period at Mar. 31, 2018	91	523,233	(1,280,586)	(55,474)	1,611,864	799,128
Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of ASU | ASC 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	43,002	43,002
Other comprehensive income, net of tax	0	0	0	4,140	0	4,140
Repurchase of Class A common stock (Note 6)	0	0	(825)	0	0	(825)
Exercise of employee stock options/vesting of stock awards	0	(4,335)	2,179	0	0	(2,156)
Stock-based compensation	0	5,041	0	0	0	5,041
Cash dividends	0	0	0	0	(20,531)	(20,531)
Balance at end of period at Mar. 31, 2019	$ 91	$ 553,270	$ (1,325,251)	$ (75,794)	$ 1,658,879	$ 811,195