Summary of Significant Accounting Policies - Allowance for Doubtful Accounts Related to Accounts Receivable (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance for doubtful accounts beginning of year	$ 32,506	$ 21,903	$ 23,842
Provision for bad debts	44,857	35,885	24,629
Allowance for Doubtful Accounts Receivable, Write-offs	(31,332)	(25,282)	(26,568)
Allowance for doubtful accounts end of year	$ 46,031	$ 32,506	$ 21,903