Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Long-term debt, carrying values	$ 1,764,835	$ 1,568,774	$ 1,700,771
Long-term debt, fair values	$ 1,876,802	$ 1,625,193	$ 1,781,152