Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Consolidated Statements of Income (Unaudited) [Abstract]
Revenue	$ 616,219	$ 499,099
Cost of sales	146,281	111,266
Gross profit	469,938	387,833
Operating expenses:
Selling expenses	257,702	209,008
General and administrative expenses	153,246	132,563
Total operating expenses	410,948	341,571
Operating income	58,990	46,262
Other income (expense), net	1,207	(4,567)
Income before provision for income taxes	60,197	41,695
Provision for income taxes	24,658	14,206
Net income	$ 35,539	$ 27,489
Net income per share (Note 2):
Basic (in dollars per share)	$ 0.66	$ 0.52
Diluted (in dollars per share)	$ 0.64	$ 0.51
Weighted-average common shares outstanding:
Basic (in shares)	53,997	52,678
Diluted (in shares)	55,959	54,057