Convertible Debentures and Credit Arrangements - Convertible Senior Debentures due 2018 Narrative (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 30, 2012
Debt Instrument [Line Items]
Amortization of debt discount		$ 2,542,000	$ 1,604,000	$ 1,472,000
Convertible Debt [Member]
Debt Instrument [Line Items]
Aggregate principal amount 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
Gain (loss) on extinguishment of debt		29,000
Long-term debt		$ 41,000,000
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock exceeded the conversion price giving the holders of the debentures an option (in days)		20 days
Number of consecutive trading days during which the closing price of the entity's common stock exceeded the conversion price for at least 20 days giving the holders of the debentures an option (in days)		30 days
Number of days after five consecutive trading days in which the trading price per $1,000 principal amount was less than 98% of the product of the closing sale price per share of Company common stock multiplied by the conversion rate on each such trading day (in days)		5 days
Number of consecutive trading days during which the trading price per $1,000 principal amount for at least five days was less than 98% of the product of the closing sale price per share of Company common stock multiplied by the conversion rate on each such trading day (in days)		5 days
Principal amount of convertible debentures		$ 1,000
Closing price is percentage of conversion price		98.00%
Conversion rate of common stock		55.17
Conversion price (in dollars per share)		$ 18.13
Closing price for common stock		$ 11.20
Sales price of common stock to conversion price		140.00%
Debentures redemption price		100.00%
Change in control due to debentures redemption		50.00%
Percentage of principal amount and accrued and unpaid interest for repurchase of debt		100.00%
Outstanding debentures		$ 41,000,000
Fair value of debentures outstanding		41,600,000
Gross carrying amount of equity component		5,600,000
Unamortized discount		500,000
Convertible principal amount outstanding		40,500,000
Amortization of debt discount		$ 1,500,000	$ 1,600,000	$ 1,500,000
Debt instrument effective rate		8.70%
Convertible Debt [Member] | Minimum [Member]
Debt Instrument [Line Items]
Percentage of the closing sales price of the entity's common stock that the conversion price exceeded giving the holders of the debentures an option (as a percent)		130.00%
Convertible Debt [Member] | Additional Paid-In Capital [Member]
Debt Instrument [Line Items]
Conversion of convertible securities, net of adjustments		$ 800,000