Long-term Borrowings - Components of Long-term Borrowings (Parenthetical) (Detail) |
6 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|
Jan. 17, 2017 |
Jul. 18, 2013 |
Jun. 30, 2018 |
Dec. 31, 2017 |
|
| 5.0% Exchangeable Senior Notes due March 2021 [Member] | Exchangeable Senior Notes [Member] | ||||
| Debt Instrument [Line Items] | ||||
| Interest rate | 5.00% | 5.00% | 5.00% | |
| Due date | Mar. 01, 2021 | Mar. 01, 2021 | Mar. 01, 2021 | |
| 6.625% Senior Notes Due 2021 [Member] | Senior Notes [Member] | ||||
| Debt Instrument [Line Items] | ||||
| Interest rate | 6.625% | 6.625% | 6.625% | |
| Due date | Jul. 15, 2021 | Jul. 15, 2021 | Jul. 15, 2021 |