Estimated Fair Values of Other Financial Instruments (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Long-term debt, including current portion, Carrying Amount	[1]	$ 800,842	$ 1,254,160
Long-term debt, including current portion, Fair Value	[1]	$ 747,818	$ 1,228,324

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