Convertible Debentures and Credit Arrangements - Convertible Senior Debentures due 2018 Narrative (Detail) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2012	Dec. 31, 2010
Debt Instrument [Line Items]
Amortization of debt discount	$ 1,085,000	$ 995,000	$ 726,000
Convertible Senior Debentures due 2018 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of convertible senior debentures				55,000,000.0
Interest rate on debentures				5.25%
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	$ 19.82
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	55,000,000
Fair value of Debentures outstanding	70,800,000
Gross carrying amount of equity component	6,400,000
Principal amount of liability component	55,000,000
Unamortized discount	4,400,000
Carrying value of liability component	50,600,000
Amortization of debt discount	1,100,000	1,000,000
Debt Instrument, Interest Rate, Effective Percentage	8.70%
Convertible Senior Debentures due 2018 [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 Senior Debentures due 2014 [Member]
Debt Instrument [Line Items]
Aggregate principal amount of convertible senior debentures					46,900,000.0
Interest rate on debentures				10.125%
Loss on retirement of debt				$ 7,900,000