Financial Instruments - Carrying And Estimated Fair Value Of Other Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Reported Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Acquisition-related contingent consideration, including current portion	$ 3,750	$ 5,692
Long-term debt	400,000	370,000
Total	403,750	375,692
Level 1 | Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Acquisition-related contingent consideration, including current portion	0	0
Long-term debt	0	0
Total	0	0
Level 2 | Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Acquisition-related contingent consideration, including current portion	0	0
Long-term debt	409,000	382,750
Total	409,000	382,750
Level 3 | Estimate of Fair Value Measurement
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Acquisition-related contingent consideration, including current portion	3,750	5,692
Long-term debt	0	0
Total	$ 3,750	$ 5,692