Summary of Significant Accounting Policies - Effect of Adoption of Standard on Condensed Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable	$ 197,973	$ 170,278
Total current assets	224,657	203,249
Total assets	1,088,092	967,315
Accounts payable	62,025	57,438
Total current liabilities	209,751	166,926
Retained earnings	419,101	339,500
Total stockholders’ equity	669,291	582,494
Total liabilities and stockholders’ equity	$ 1,088,092	967,315
Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers | As Originally Reported
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable		170,610
Total current assets		203,581
Total assets		967,647
Accounts payable		57,717
Total current liabilities		167,205
Retained earnings		339,553
Total stockholders’ equity		582,547
Total liabilities and stockholders’ equity		967,647
Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers | Effect of Change
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable		(332)
Total current assets		(332)
Total assets		(332)
Accounts payable		(279)
Total current liabilities		(279)
Retained earnings		(53)
Total stockholders’ equity		(53)
Total liabilities and stockholders’ equity		$ (332)