Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Long-term debt, carrying values	$ 1,722,540	$ 1,686,189	$ 1,764,835
Long-term debt, fair values	$ 1,830,750	$ 1,752,338	$ 1,876,802