Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Fair Value Disclosures [Abstract]
Long-term debt, carrying values	$ 1,781,981	$ 1,686,189	$ 1,779,217
Long-term debt, fair values	$ 1,885,231	$ 1,752,338	$ 1,890,319