Consolidated Statements of Stockholders' Equity - 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
Balance at beginning of period at Dec. 31, 2015	$ 91	$ 419,921	$ (1,017,063)	$ (71,269)	$ 1,493,941	$ 825,621
Stockholders' Equity [Roll Forward]
Net income	0	0	0	0	143,086	143,086
Other comprehensive income, net of tax	0	0	0	(12,853)	0	(12,853)
Repurchase of Class A common stock (Note 8)	0	0	(247,208)	0	0	(247,208)
Exercise of employee stock options/vesting of stock awards	0	159	14,148	0	0	14,307
Excess tax benefit from equity awards	0	3,840	0	0	0	3,840
Stock-based compensation	0	8,890	0	0	0	8,890
Equity component of convertible note issuance (net)	0	6,825	0	0	0	6,825
Cash dividends	0	0	0	0	(78,438)	(78,438)
Balance at end of period at Dec. 31, 2016	91	439,635	(1,250,123)	(84,122)	1,558,589	664,070
Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of ASU | ASU 2016-09 [Member]	0	2,800	0	0	(2,800)	0
Net income	0	0	0	0	129,437	129,437
Other comprehensive income, net of tax	0	0	0	17,804	0	17,804
Repurchase of Class A common stock (Note 8)	0	0	(71,731)	0	0	(71,731)
Exercise of employee stock options/vesting of stock awards	0	9,479	14,964	0	0	24,443
Stock-based compensation	0	19,314	0	0	0	19,314
Acquisition of noncontrolling interests	0	(11,067)	0	0	0	(11,067)
Acquisitions	0	6,188	2,196	0	0	8,384
Cash dividends	0	0	0	0	(76,058)	(76,058)
Balance at end of period at Dec. 31, 2017	91	466,349	(1,304,694)	(66,318)	1,609,168	704,596
Stockholders' Equity [Roll Forward]
Cumulative effect adjustment from adoption of ASU | ASC 606 [Member]	0	0	0	0	(13,042)	(13,042)
Cumulative effect adjustment from adoption of ASU | ASU 2018-02 [Member]	0	0	0	0	(1,681)	(1,681)
Net income	0	0	0	0	121,887	121,887
Other comprehensive income, net of tax	0	0	0	(13,616)	0	(13,616)
Repurchase of Class A common stock (Note 8)	0	0	(69,565)	0	0	(69,565)
Exercise of employee stock options/vesting of stock awards	0	2,804	7,973	0	0	10,777
Stock-based compensation	0	26,609	0	0	0	26,609
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	(80,581)	(80,581)
Balance at end of period at Dec. 31, 2018	$ 91	$ 552,564	$ (1,326,605)	$ (79,934)	$ 1,635,751	$ 781,867