Financial Instruments - Additional Information (Detail) - USD ($)	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Fair Value Disclosures [Abstract]
Long-term debt, carrying values	$ 1,658,001,000	$ 1,585,395,000	$ 1,084,801,000
Long-term debt, fair values	$ 1,729,511,000	$ 1,680,584,000	$ 1,168,302,000