Credit Facility and Convertible Debentures - Convertible Senior Debentures Narrative (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Nov. 30, 2012
Debt Instrument [Line Items]
Convertible senior debentures - current	$ 40,829,000		$ 40,485,000
Interest expense	2,690,000	$ 1,198,000
Convertible Debt
Debt Instrument [Line Items]
Aggregate face value of convertible senior debentures	41,000,000			$ 55,000,000.0
Interest rate on debentures				5.25%
Repurchased face amount	14,000,000
Repayments of long-term debt	14,500,000
Long-term debt			$ 41,000,000
Gross carrying amount of equity component	5,600,000
Principal amount outstanding	41,000,000
Deferred finance costs, noncurrent, net	200,000
Convertible senior debentures - current	40,800,000
Interest expense	900,000	1,100,000
Interest paid	$ 0	$ 0
Debt instrument, interest rate, effective percentage	8.70%
Fair value of debentures outstanding	$ 41,300,000