Convertible Debentures and Credit Arrangements - Convertible Senior Debentures Narrative (Detail) - Convertible Senior Debentures due 2018 [Member] - USD ($)	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Nov. 30, 2012
Debt Instrument [Line Items]
Aggregate face value of convertible senior debentures	$ 55,000,000		$ 55,000,000.0
Interest rate on debentures			5.25%
Fair value of debentures outstanding	68,400,000
Gross carrying amount of equity component	6,400,000
Unamortized discount	3,900,000
Carrying value of liability component	51,100,000
Amortization of debt discount (premium)	$ 600,000	$ 500,000
Debt instrument, interest rate, effective percentage	8.70%