Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Long-term debt, carrying values	$ 3,028,108	$ 3,027,203	$ 1,846,294
Long-term debt, fair values	$ 3,037,069	$ 3,144,902	$ 1,937,310