Summary of Significant Accounting Policies (Impact of New Accounting Principles, Income) (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies [Abstract]
Revenue	$ 170,632	$ 190,061	$ 187,103	$ 165,876	$ 145,844	$ 139,239	$ 117,324	$ 98,596	$ 713,672	$ 501,003	$ 301,563
Cost of sales									176,814	121,104	73,751
Gross profit	128,068	142,933	140,710	125,147	109,091	107,201	88,799	74,808	536,858	379,899	227,812
Selling, general, and administrative									445,819	310,836	188,180
Income from operations									91,039	69,063	39,632
Other income (expense)
Interest income, net									1,295	1,306	558
Other income (expense)									29	179	136
Other income (expense)									1,324	1,485	694
Income from operations before income taxes	18,981	20,583	27,778	25,021	20,170	17,828	17,619	14,931	92,363	70,548	40,326
Provision for income taxes									14,447	14,759	12,605
Net income	$ 19,881	$ 15,902	$ 21,383	$ 20,750	$ 15,653	$ 13,781	$ 14,133	$ 12,222	$ 77,916	$ 55,789	$ 27,721
Basic earnings per share
Earnings per share, basic	$ 1.71	$ 1.36	$ 1.80	$ 1.75	$ 1.32	$ 1.15	$ 1.17	$ 1.02	$ 6.62	$ 4.67	$ 2.32
Diluted earnings per share
Earnings per share, diluted	$ 1.66	$ 1.32	$ 1.75	$ 1.70	$ 1.30	$ 1.14	$ 1.16	$ 1.01	$ 6.43	$ 4.62	$ 2.29
Weighted average shares outstanding -
Basic									11,771	11,947	11,924
Diluted									12,117	12,079	12,088