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