Long-Term Debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 04, 2015 USD ($) $ / shares	Jun. 28, 2014 USD ($)	Jul. 04, 2015 USD ($) $ / shares	Jun. 28, 2014 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Long-term debt	$ 427,294		$ 427,294		$ 454,922
Exchangeable unsecured notes, due 2102	38,642		38,642		38,642
Credit facility	170,000		170,000		200,000
Less current portion	0		0		0
Long-term debt, less current portion	427,294		427,294		454,922
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%
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 575,000		$ 575,000		575,000
Unamortized discount	(357,735)		(357,735)		(359,825)
Embedded derivative	1,387		1,387		1,105
Carrying value of liability component	218,652		218,652		216,280
Equity component - net carrying value	230,214		230,214		230,214
Interest expense related to debentures [Abstract]
Contractual coupon interest	3,235	$ 3,235	6,470	$ 6,470
Non-cash amortization of debt discount	1,054	975	2,090	1,933
Non-cash amortization of deferred financing costs	48	49	95	95
Non-cash change in value of derivative liability	335	(117)	282	(38)
Total interest expense related to the debentures	4,672	4,142	8,937	8,460
Convertible Senior Debentures, Due 2040 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	104,986		$ 104,986		103,841
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 Instrument, Convertible, Stock Price Trigger | $ / shares			$ 17.60
Debt conversion rate	73.8821
Debt effective conversion price | $ / shares	$ 13.54		$ 13.54
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	(170,646)		(170,646)		(171,685)
Embedded derivative	632		632		526
Carrying value of liability component	104,986		104,986		103,841
Equity component - net carrying value	110,094		110,094		110,094
Interest expense related to debentures [Abstract]
Contractual coupon interest	1,547	1,547	3,094	3,094
Non-cash amortization of debt discount	524	485	1,039	961
Non-cash amortization of deferred financing costs	22	22	44	44
Non-cash change in value of derivative liability	165	(76)	106	(31)
Total interest expense related to the debentures	2,258	1,978	4,283	4,068
Convertible Senior Debentures, Due 2041 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	53,914		$ 53,914		53,249
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 Instrument, Convertible, Stock Price Trigger | $ / shares			$ 24.12
Debt conversion rate	53.9153
Debt effective conversion price | $ / shares	$ 18.55		$ 18.55
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	(96,564)		(96,564)		(97,092)
Embedded derivative	478		478		341
Carrying value of liability component	53,914		53,914		53,249
Equity component - net carrying value	62,246		62,246		62,246
Interest expense related to debentures [Abstract]
Contractual coupon interest	844	844	1,688	1,688
Non-cash amortization of debt discount	267	245	528	486
Non-cash amortization of deferred financing costs	12	13	24	24
Non-cash change in value of derivative liability	124	(20)	137	(6)
Total interest expense related to the debentures	1,247	1,082	2,377	2,192
Convertible Senior Debentures, Due 2042 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	59,752		$ 59,752		59,190
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 Instrument, Convertible, Stock Price Trigger | $ / shares			$ 14.96
Debt conversion rate	86.8677
Debt effective conversion price | $ / shares	$ 11.51		$ 11.51
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	(90,525)		(90,525)		(91,048)
Embedded derivative	277		277		238
Carrying value of liability component	59,752		59,752		59,190
Equity component - net carrying value	57,874		57,874		$ 57,874
Interest expense related to debentures [Abstract]
Contractual coupon interest	844	844	1,688	1,688
Non-cash amortization of debt discount	263	245	523	486
Non-cash amortization of deferred financing costs	14	14	27	27
Non-cash change in value of derivative liability	46	(21)	39	(1)
Total interest expense related to the debentures	$ 1,167	$ 1,082	$ 2,277	$ 2,200