Derivative Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	$ 5,299,000
Zero Cost Collars [Member] | Nomura Financial Investment (Korea) Co., Ltd. [Member]
Derivative [Line Items]
Deposit with counterparty		$ 2,500,000
Cash collateral for credit exposure in derivatives		$ 650,000
Zero Cost Collar and Forward Contracts [Member] | Nomura Financial Investment (Korea) Co., Ltd. [Member]
Derivative [Line Items]
Deposit with counterparty	7,600,000
Threshold amount of cash collateral	500,000
Cash collateral for credit exposure in derivatives	0
Termination provisions for cash and cash equivalents	$ 30,000,000