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
	Mar. 31, 2018	Mar. 31, 2017
Administrative expenses
Sales and marketing	$ 32,352	$ 25,579
General and administrative	32,657	15,250
Operating income	51,020	52,510
Provision for income taxes	10,890	18,654
Net income	$ 41,160	$ 33,694
Earnings per share, basic	$ 0.71	$ 0.58
Earnings per share, diluted	$ 0.70	$ 0.57
As Previously Reported [Member]
Administrative expenses
Sales and marketing		$ 36,848
General and administrative		17,826
Operating income		38,665
Provision for income taxes		12,889
Net income		$ 25,614
Earnings per share, basic		$ 0.44
Earnings per share, diluted		$ 0.43
Adjustments [Member]
Administrative expenses
Sales and marketing		$ (11,269)
General and administrative		(2,576)
Operating income		13,845
Provision for income taxes		5,765
Net income		$ 8,080
Earnings per share, basic		$ 0.14
Earnings per share, diluted		$ 0.14