Estimated Fair Values of Other Financial Instruments (Detail) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Long-term debt, including current portion, Carrying Amount	[1]	$ 648,058	$ 675,229
Long-term debt, including current portion, Fair Value	[1]	$ 554,677	$ 582,370

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