Long-Term Debt (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Credit facility	$ 184,000		$ 150,000
Deferred Financing Costs	(8,656)		(9,101)
Long-term debt	406,385		370,470
Less current portion	0		0
Long-term debt, less current portion	$ 406,385		370,470
Debt Instruments [Abstract]
Debt instrument percentage of conversion price (in hundredths)	130.00%
Debt instrument, Percentage of sale price of common stock (in hundredths)	98.00%
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 575,000		575,000
Unamortized discount	(344,658)		(345,967)
Embedded derivative	699		538
Carrying value of liability component	231,041		229,571
Equity component - net carrying value	230,214		230,214
Interest expense related to debentures [Abstract]
Contractual coupon interest	3,235	$ 3,235
Non-cash amortization of debt discount	1,309	1,211
Non-cash amortization of deferred financing costs	47	47
Non-cash change in value of derivative liability	161	(27)
Total interest expense related to the debentures	4,752	4,466
Convertible Senior Debentures, Due 2040 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 111,157		110,412
Debt Instruments [Abstract]
Issuance date	Nov. 09, 2010
Debt maturity date	Nov. 15, 2040
Stated rate of interest on debt (in hundredths)	2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	8.00%
Debt conversion rate	77.4680
Debt effective conversion price	$ 12.91
Debt Instrument, Convertible, Stock Price Trigger	$ 16.78
Convertible senior debentures call date	Nov. 20, 2020
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 275,000		275,000
Unamortized discount	(164,143)		(164,794)
Embedded derivative	300		206
Carrying value of liability component	111,157		110,412
Equity component - net carrying value	110,094		110,094
Interest expense related to debentures [Abstract]
Contractual coupon interest	1,547	1,547
Non-cash amortization of debt discount	651	602
Non-cash amortization of deferred financing costs	22	22
Non-cash change in value of derivative liability	94	(25)
Total interest expense related to the debentures	2,314	2,146
Convertible Senior Debentures, Due 2041 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 57,031		56,641
Debt Instruments [Abstract]
Issuance date	May 13, 2011
Debt maturity date	May 15, 2041
Stated rate of interest on debt (in hundredths)	2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	8.375%
Debt conversion rate	56.5321
Debt effective conversion price	$ 17.69
Debt Instrument, Convertible, Stock Price Trigger	$ 23.00
Convertible senior debentures call date	May 20, 2021
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 150,000		150,000
Unamortized discount	(93,239)		(93,573)
Embedded derivative	270		214
Carrying value of liability component	57,031		56,641
Equity component - net carrying value	62,246		62,246
Interest expense related to debentures [Abstract]
Contractual coupon interest	844	844
Non-cash amortization of debt discount	334	308
Non-cash amortization of deferred financing costs	12	12
Non-cash change in value of derivative liability	56	2
Total interest expense related to the debentures	1,246	1,166
Convertible Senior Debentures, Due 2042 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 62,853		62,518
Debt Instruments [Abstract]
Issuance date	May 31, 2012
Debt maturity date	Jun. 01, 2042
Stated rate of interest on debt (in hundredths)	2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	7.50%
Debt conversion rate	91.0838
Debt effective conversion price	$ 10.98
Debt Instrument, Convertible, Stock Price Trigger	$ 14.27
Convertible senior debentures call date	Jun. 07, 2022
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 150,000		150,000
Unamortized discount	(87,276)		(87,600)
Embedded derivative	129		118
Carrying value of liability component	62,853		62,518
Equity component - net carrying value	57,874		$ 57,874
Interest expense related to debentures [Abstract]
Contractual coupon interest	844	844
Non-cash amortization of debt discount	324	301
Non-cash amortization of deferred financing costs	13	13
Non-cash change in value of derivative liability	11	(4)
Total interest expense related to the debentures	$ 1,192	$ 1,154