Long-Term Debt (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 28, 2014	Jun. 29, 2013	Jun. 28, 2014	Jun. 29, 2013	Dec. 31, 2013
Debt Instrument [Line Items]
Long-term debt	$ 386,806		$ 386,806		$ 364,911
Exchangeable unsecured notes, due 2102	38,642		38,642		38,642
Credit facility	134,000		134,000		114,000
Less current portion	0		0		0
Long-term debt, less current portion	386,806		386,806		364,911
Debt Instruments [Abstract]
Principal amount of debt	575,000		575,000		575,000
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	575,000		575,000		575,000
Unamortized discount	(361,835)		(361,835)		(363,768)
Embedded derivative	999		999		1,037
Carrying value of liability component	214,164		214,164		212,269
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	975	901	1,933	1,787
Non-cash amortization of deferred financing costs	49	48	95	95
Non-cash change in value of derivative liability	(117)	(18)	(38)	(326)
Total interest expense related to the debentures	4,142	4,166	8,460	8,026
Convertible Senior Debentures 2040 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	102,776		102,776		101,846
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			$ 17.90
Debt conversion rate	72.6207
Debt effective conversion price	$ 13.77		$ 13.77
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	(172,684)		(172,684)		(173,645)
Embedded derivative	460		460		491
Carrying value of liability component	102,776		102,776		101,846
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	485	448	961	888
Non-cash amortization of deferred financing costs	22	22	44	44
Non-cash change in value of derivative liability	(76)	(16)	(31)	(163)
Total interest expense related to the debentures	1,978	2,001	4,068	3,863
Convertible Senior Debentures 2041 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	52,744		52,744		52,264
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			$ 24.53
Debt conversion rate	52.9947
Debt effective conversion price	$ 18.87		$ 18.87
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	(97,599)		(97,599)		(98,085)
Embedded derivative	343		343		349
Carrying value of liability component	52,744		52,744		52,264
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	245	226	486	448
Non-cash amortization of deferred financing costs	13	12	24	24
Non-cash change in value of derivative liability	(20)	1	(6)	(87)
Total interest expense related to the debentures	1,082	1,083	2,192	2,073
Convertible Senior Debentures 2042 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	58,644		58,644		58,159
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			$ 15.22
Debt conversion rate	85.3846
Debt effective conversion price	$ 11.71		$ 11.71
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	(91,552)		(91,552)		(92,038)
Embedded derivative	196		196		197
Carrying value of liability component	58,644		58,644		58,159
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	245	227	486	451
Non-cash amortization of deferred financing costs	14	14	27	27
Non-cash change in value of derivative liability	(21)	(3)	(1)	(76)
Total interest expense related to the debentures	$ 1,082	$ 1,082	$ 2,200	$ 2,090