Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Consolidated Statements of Income (Unaudited) [Abstract]
Revenue	$ 675,312	$ 563,698	$ 1,995,721	$ 1,612,898
Cost of sales	157,457	120,832	472,287	353,619
Gross profit	517,855	442,866	1,523,434	1,259,279
Operating expenses:
Selling expenses	271,509	235,285	801,968	672,646
General and administrative expenses	165,671	143,219	499,037	411,270
Total operating expenses	437,180	378,504	1,301,005	1,083,916
Operating income	80,675	64,362	222,429	175,363
Other income (expense), net	(6,982)	(1,172)	(16,940)	(8,470)
Income before provision for income taxes	73,693	63,190	205,489	166,893
Provision for income taxes	20,547	21,518	65,843	55,691
Net income	$ 53,146	$ 41,672	$ 139,646	$ 111,202
Net income per share (Note 2):
Basic (in dollars per share)	$ 0.96	$ 0.79	$ 2.54	$ 2.10
Diluted (in dollars per share)	$ 0.94	$ 0.76	$ 2.47	$ 2.04
Weighted-average common shares outstanding:
Basic (in shares)	55,603	52,873	55,075	52,834
Diluted (in shares)	56,704	54,834	56,539	54,519