Estimated Fair Values of Other Financial Instruments (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Long-term debt, including current portion, Carrying Amount	[1]	$ 691,638	$ 800,842
Long-term debt, including current portion, Fair Value	[1]	$ 685,810	$ 747,818

[1] Carrying amounts include aggregate unamortized debt discount and debt issuance costs.