Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Consolidated Statements of Income (Unaudited) [Abstract]
Revenue	$ 703,347	$ 589,934	$ 1,833,741	$ 1,837,057
Cost of sales	183,374	140,162	463,277	440,854
Gross profit	519,973	449,772	1,370,464	1,396,203
Operating expenses:
Selling expenses	280,695	231,937	735,365	727,473
General and administrative expenses	165,050	147,943	466,232	455,983
Total operating expenses	445,745	379,880	1,201,597	1,183,456
Operating income	74,228	69,892	168,867	212,747
Other income (expense), net	525	(4,979)	(4,068)	(11,153)
Income before provision for income taxes	74,753	64,913	164,799	201,594
Provision for income taxes	18,446	20,823	46,911	68,153
Net income	$ 56,307	$ 44,090	$ 117,888	$ 133,441
Net income per share (Note 6):
Basic (in dollars per share)	$ 1.10	$ 0.79	$ 2.24	$ 2.40
Diluted (in dollars per share)	$ 1.08	$ 0.79	$ 2.23	$ 2.39
Weighted-average common shares outstanding:
Basic (in shares)	51,308	55,548	52,741	55,507
Diluted (in shares)	52,243	55,788	52,906	55,950