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
|