Convertible Debentures and Credit Arrangements - Convertible Senior Debentures Narrative (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Nov. 30, 2012
Debt Instrument [Line Items]
Deferred finance costs, noncurrent, net					$ 800,000
Convertible Senior Debentures due 2018
Debt Instrument [Line Items]
Aggregate face value of convertible senior debentures	$ 55,000,000		$ 55,000,000			$ 55,000,000.0
Interest rate on debentures						5.25%
Fair value of debentures outstanding	55,500,000		55,500,000
Gross carrying amount of equity component	6,400,000		6,400,000
Unamortized discount	2,600,000		2,600,000
Deferred finance costs, noncurrent, net	600,000		600,000
Carrying value of liability component	51,700,000		51,700,000
Amortization of debt discount (premium)	$ 300,000	$ 300,000	$ 600,000	$ 600,000
Debt instrument, interest rate, effective percentage	8.70%		8.70%