Derivative Financial Instruments - Additional Information (Detail) - USD ($) |
6 Months Ended | |
|---|---|---|
Jun. 30, 2018 |
Dec. 31, 2017 |
|
| Derivative [Line Items] | ||
| Estimated amount reclassified from accumulated other comprehensive income into income, period | 12 months | |
| Estimated amount reclassified from accumulated other comprehensive income into income | $ 1,280,000 | |
| Zero Cost Collar and Forward Contracts [Member] | Nomura Financial Investment (Korea) Co., Ltd. [Member] | ||
| Derivative [Line Items] | ||
| Deposit with counterparty | 10,100,000 | $ 7,600,000 |
| Threshold amount of cash collateral | 500,000 | |
| Cash collateral for credit exposure in derivatives | 0 | $ 0 |
| Termination provisions for cash and cash equivalents | $ 30,000,000 |