Summary of Significant Accounting Policies - Summary of Reflection of New Adopted Standards in Selected Consolidated Statement of Operations Line Items (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Administrative expenses
Sales and marketing	$ 31,647	$ 27,430	$ 63,999	$ 53,009
General and administrative	18,995	23,594	51,652	38,844
Operating income	43,037	18,792	94,057	71,302
Provision for income taxes	7,796	(1,356)	18,686	17,298
Net income	$ 35,722	$ 20,016	$ 76,882	$ 53,710
Earnings per share, basic	$ 0.62	$ 0.34	$ 1.33	$ 0.93
Earnings per share, diluted	$ 0.61	$ 0.34	$ 1.31	$ 0.91
As Previously Reported [Member]
Administrative expenses
Sales and marketing		$ 34,070		$ 70,918
General and administrative		26,657		44,483
Operating income		9,089		47,754
Provision for income taxes		(5,264)		7,625
Net income		$ 14,221		$ 39,835
Earnings per share, basic		$ 0.24		$ 0.69
Earnings per share, diluted		$ 0.24		$ 0.67
Adjustments [Member]
Administrative expenses
Sales and marketing		$ (6,640)		$ (17,909)
General and administrative		(3,063)		(5,639)
Operating income		9,703		23,548
Provision for income taxes		3,908		9,673
Net income		$ 5,795		$ 13,875
Earnings per share, basic		$ 0.10		$ 0.24
Earnings per share, diluted		$ 0.10		$ 0.24