Fair Value Measurements - Carrying Amounts and Fair Values of Company's Financial Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion as reflected on balance sheets	$ 2,186,326	$ 1,930,731
Fair value, Long-term debt, including current portion	2,155,670	1,937,671
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, Long-term debt, including current portion	0	0
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, Long-term debt, including current portion	2,155,670	1,937,671
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value, Long-term debt, including current portion	$ 0	0
Long-term debt excluding revolving credit facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, including current portion as reflected on balance sheets		$ 1,930,731