Long-Term Debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 29, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 29, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($)	Jun. 08, 2018 $ / shares	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Credit facility	$ 0			$ 0			$ 150,000
Deferred Financing Costs	(17,716)			(17,716)			(9,101)
Long-term debt	588,586			588,586			370,470
Less current portion	0			0			0
Long-term debt, less current portion	$ 588,586			$ 588,586			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%			98.00%
Purchase price of extinguished debt		$ 584,991
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 285,940			$ 285,940			575,000
Unamortized discount	(171,845)			(171,845)			(345,967)
Embedded derivative	354			354			538
Carrying value of liability component	114,449			114,449			229,571
Equity component - net carrying value	115,494			115,494			230,214
Interest expense related to debentures [Abstract]
Non-cash amortization of debt discount				6,966	$ 3,703
Stock price per share (in dollars per share) | $ / shares						$ 24.70
Allocated liability component of repurchased debt		133,647
Allocated equity component of repurchased debt		451,344
Convertible Senior Debentures, Due 2040 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 22,486			$ 22,486			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%			2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	8.00%			8.00%
Debt conversion rate	78.0506
Debt effective conversion price | $ / shares	$ 12.81			$ 12.81
Debt Instrument, Convertible, Stock Price Trigger | $ / shares				$ 16.65
Convertible senior debentures call date	Nov. 20, 2020
Principal amount of repurchased debt		220,000
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 55,000			$ 55,000			275,000
Unamortized discount	(32,561)			(32,561)			(164,794)
Embedded derivative	47			47			206
Carrying value of liability component	22,486			22,486			110,412
Equity component - net carrying value	22,019			22,019			110,094
Interest expense related to debentures [Abstract]
Contractual coupon interest	310		$ 1,547	2,991	4,641
Non-cash amortization of debt discount	135		626	1,269	1,841
Non-cash amortization of deferred financing costs	4		22	42	66
Non-cash change in value of derivative liability	20		(53)	81	(97)
Total interest expense related to the debentures	469		2,142	4,383	6,451
Net carrying value of repurchased debt		89,276
Convertible Senior Debentures, Due 2041 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 57,695			$ 57,695			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%			2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	8.375%			8.375%
Debt conversion rate	56.9573
Debt effective conversion price | $ / shares	$ 17.56			$ 17.56
Debt Instrument, Convertible, Stock Price Trigger | $ / shares				$ 22.83
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	(92,549)			(92,549)			(93,573)
Embedded derivative	244			244			214
Carrying value of liability component	57,695			57,695			56,641
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	349		321	1,024	943
Non-cash amortization of deferred financing costs	12		11	36	35
Non-cash change in value of derivative liability	71		(21)	30	(13)
Total interest expense related to the debentures	1,276		1,155	3,622	3,497
Convertible Senior Debentures, Due 2042 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 34,268			$ 34,268			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%			2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	7.50%			7.50%
Debt conversion rate	91.7689
Debt effective conversion price | $ / shares	$ 10.90			$ 10.90
Debt Instrument, Convertible, Stock Price Trigger | $ / shares				$ 14.17
Convertible senior debentures call date	Jun. 07, 2022
Principal amount of repurchased debt		69,060
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 80,940			$ 80,940			150,000
Unamortized discount	(46,735)			(46,735)			(87,600)
Embedded derivative	63			63			118
Carrying value of liability component	34,268			34,268			62,518
Equity component - net carrying value	31,229			31,229			57,874
Interest expense related to debentures [Abstract]
Contractual coupon interest	455		844	2,013	2,532
Non-cash amortization of debt discount	184		312	783	919
Non-cash amortization of deferred financing costs	7		13	31	40
Non-cash change in value of derivative liability	19		(21)	4	(13)
Total interest expense related to the debentures	665		1,148	2,831	3,478
Net carrying value of repurchased debt		$ 29,037
Convertible Senior Notes, Due 2025 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 491,853			$ 491,853			$ 0
Debt Instruments [Abstract]
Debt maturity date		Jun. 15, 2025
Stated rate of interest on debt (in hundredths)	2.25%			2.25%
Effective rate of interest on convertible senior debentures (in hundredths)	5.50%			5.50%
Debt conversion rate	31.7536
Debt effective conversion price | $ / shares	$ 31.49			$ 31.49
Debt Instrument, Convertible, Stock Price Trigger | $ / shares	$ 40.94
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	$ 600,000			$ 600,000
Unamortized discount	(108,147)			(108,147)
Carrying value of liability component	491,853			491,853
Equity component - net carrying value	85,262			85,262
Interest expense related to debentures [Abstract]
Contractual coupon interest	3,375			4,088
Non-cash amortization of debt discount	3,334			3,890
Non-cash amortization of deferred financing costs	454			605
Total interest expense related to the debentures	7,163			8,583
Convertible senior debentures [Member]
Interest expense related to debentures [Abstract]
Contractual coupon interest	1,609		3,235	7,536	9,705
Non-cash amortization of debt discount	668		1,259	3,076	3,703
Non-cash amortization of deferred financing costs	23		46	109	141
Non-cash change in value of derivative liability	110		(95)	115	(123)
Total interest expense related to the debentures	$ 2,410		$ 4,445	$ 10,836	$ 13,426