Valuation and Qualifying Accounts and Reserves (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 4,310	$ 2,472	$ 1,563
Additions (reductions) to Costs and Expenses	9,948	8,585	8,944
Write offs	(7,080)	(6,747)	(8,035)
Balance at End of Period	7,178	4,310	2,472
Allowance for deferred tax assets
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	256	31,685	32,498
Additions (reductions) to Costs and Expenses	21	(31,429)	(813)
Write offs	0	0	0
Balance at End of Period	$ 277	$ 256	$ 31,685