Credit Facility and Convertible Debentures - Convertible Senior Debentures Narrative (Detail) - USD ($)		2 Months Ended	3 Months Ended		9 Months Ended
	May 15, 2018	Jul. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Nov. 30, 2012
Debt Instrument [Line Items]
Interest expense			$ 3,310,000	$ 2,643,000	$ 9,422,000	$ 5,953,000
2018 Debentures
Debt Instrument [Line Items]
Repayments of debt	$ 41,000,000
Convertible Debt
Debt Instrument [Line Items]
Aggregate face value of convertible senior debentures								$ 55,000,000.0
Interest rate on debentures								5.25%
Repurchased face amount		$ 14,000,000
Repayments of long-term debt		$ 14,500,000
Interest expense			0	900,000	1,300,000	3,200,000
Debt instrument, interest rate, effective percentage							8.70%
Interest paid			$ 0	$ 0	$ 1,100,000	$ 1,500,000