Long-Term Debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares	Oct. 01, 2016 USD ($)	Sep. 30, 2017 USD ($) $ / shares	Oct. 01, 2016 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Credit facility	$ 138,000		$ 138,000		$ 143,000
Deferred Financing Costs	(9,545)		(9,545)		(10,880)
Long-term debt	356,938		356,938		357,023
Less current portion	0		0		0
Long-term debt, less current portion	356,938		356,938		357,023
Debt Instruments [Abstract]
Principal amount of debt	$ 575,000		$ 575,000		575,000
Debt instrument percentage of conversion price (in hundredths)			130.00%
Debt instrument, Percentage of sale price of common stock (in hundredths)	98.00%		98.00%
Principal payments on long-term debt and capital leases			$ 0	$ (34,044)
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 575,000		575,000		575,000
Unamortized discount	(347,248)		(347,248)		(350,951)
Embedded derivative	731		731		854
Carrying value of liability component	228,483		228,483		224,903
Equity component - net carrying value	230,214		230,214		230,214
Interest expense related to debentures [Abstract]
Contractual coupon interest	3,235	$ 3,235	9,705	9,705
Non-cash amortization of debt discount	1,259	1,166	3,703	3,425
Non-cash amortization of deferred financing costs	46	46	141	141
Non-cash change in value of derivative liability	(95)	(288)	(123)	(388)
Total interest expense related to the debentures	4,445	4,159	13,426	12,883
Convertible Senior Debentures, Due 2040 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	109,864		$ 109,864		108,120
Debt Instruments [Abstract]
Issuance date			Nov. 09, 2010
Debt maturity date			Nov. 15, 2040
Principal amount of debt	$ 275,000		$ 275,000		275,000
Stated rate of interest on debt (in hundredths)	2.25%		2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	8.00%		8.00%
Debt conversion rate	76.9650
Debt effective conversion price | $ / shares	$ 12.99		$ 12.99
Debt Instrument, Convertible, Stock Price Trigger | $ / shares			$ 16.89
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		275,000
Unamortized discount	(165,432)		(165,432)		(167,273)
Embedded derivative	296		296		393
Carrying value of liability component	109,864		109,864		108,120
Equity component - net carrying value	110,094		110,094		110,094
Interest expense related to debentures [Abstract]
Contractual coupon interest	1,547	1,547	4,641	4,641
Non-cash amortization of debt discount	626	579	1,841	1,702
Non-cash amortization of deferred financing costs	22	22	66	66
Non-cash change in value of derivative liability	(53)	(115)	(97)	(150)
Total interest expense related to the debentures	2,142	2,033	6,451	6,259
Convertible Senior Debentures, Due 2041 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	56,372		$ 56,372		55,442
Debt Instruments [Abstract]
Issuance date			May 13, 2011
Debt maturity date			May 15, 2041
Principal amount of debt	$ 150,000		$ 150,000		150,000
Stated rate of interest on debt (in hundredths)	2.25%		2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	8.375%		8.375%
Debt conversion rate	56.1650
Debt effective conversion price | $ / shares	$ 17.80		$ 17.80
Debt Instrument, Convertible, Stock Price Trigger | $ / shares			$ 23.14
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		150,000
Unamortized discount	(93,900)		(93,900)		(94,843)
Embedded derivative	272		272		285
Carrying value of liability component	56,372		56,372		55,442
Equity component - net carrying value	62,246		62,246		62,246
Interest expense related to debentures [Abstract]
Contractual coupon interest	844	844	2,532	2,532
Non-cash amortization of debt discount	321	296	943	869
Non-cash amortization of deferred financing costs	11	11	35	35
Non-cash change in value of derivative liability	(21)	(102)	(13)	(136)
Total interest expense related to the debentures	1,155	1,049	3,497	3,300
Convertible Senior Debentures, Due 2042 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	62,247		$ 62,247		61,341
Debt Instruments [Abstract]
Issuance date			May 31, 2012
Debt maturity date			Jun. 01, 2042
Principal amount of debt	$ 150,000		$ 150,000		150,000
Stated rate of interest on debt (in hundredths)	2.25%		2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	7.50%		7.50%
Debt conversion rate	90.4924
Debt effective conversion price | $ / shares	$ 11.05		$ 11.05
Debt Instrument, Convertible, Stock Price Trigger | $ / shares			$ 14.37
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		150,000
Unamortized discount	(87,916)		(87,916)		(88,835)
Embedded derivative	163		163		176
Carrying value of liability component	62,247		62,247		61,341
Equity component - net carrying value	57,874		57,874		$ 57,874
Interest expense related to debentures [Abstract]
Contractual coupon interest	844	844	2,532	2,532
Non-cash amortization of debt discount	312	291	919	854
Non-cash amortization of deferred financing costs	13	13	40	40
Non-cash change in value of derivative liability	(21)	(71)	(13)	(102)
Total interest expense related to the debentures	$ 1,148	$ 1,077	$ 3,478	$ 3,324