Long-Term Debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares	Jul. 01, 2017 USD ($)	Apr. 02, 2016 $ / shares	Jun. 30, 2018 USD ($) $ / shares	Jul. 01, 2017 USD ($)	Jun. 08, 2018 $ / shares	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Credit facility	$ 96,000			$ 96,000			$ 150,000
Deferred Financing Costs	(18,591)			(18,591)			(9,101)
Long-term debt	679,598			679,598			370,470
Less current portion	0			0			0
Long-term debt, less current portion	$ 679,598			$ 679,598			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			$ 584,991
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	285,940			285,940			575,000
Unamortized discount	(172,513)			(172,513)			(345,967)
Embedded derivative	244			244			538
Carrying value of liability component	113,671			113,671			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				2,964	$ 2,444
Stock price per share (in dollars per share) | $ / shares						$ 24.70
Allocated liability component of repurchased debt	133,647			133,647
Allocated equity component of repurchased debt	451,344			451,344
Convertible Senior Debentures, Due 2040 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 22,331			$ 22,331			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	77.7399
Debt effective conversion price | $ / shares	$ 12.86			$ 12.86
Debt Instrument, Convertible, Stock Price Trigger | $ / shares				$ 16.72
Convertible senior debentures call date	Nov. 20, 2020
Principal amount of repurchased debt	$ 220,000			$ 220,000
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	55,000			55,000			275,000
Unamortized discount	(32,696)			(32,696)			(164,794)
Embedded derivative	27			27			206
Carrying value of liability component	22,331			22,331			110,412
Equity component - net carrying value	22,019			22,019			110,094
Interest expense related to debentures [Abstract]
Contractual coupon interest	1,134	$ 1,547		2,681	3,094
Non-cash amortization of debt discount	483	613		1,134	1,215
Non-cash amortization of deferred financing costs	16	22		38	44
Non-cash change in value of derivative liability	(33)	(19)		61	(44)
Total interest expense related to the debentures	1,600	2,163		3,914	4,309
Net carrying value of repurchased debt	89,276			89,276
Convertible Senior Debentures, Due 2041 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 57,275			$ 57,275			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	56.7305
Debt effective conversion price | $ / shares	$ 17.63			$ 17.63
Debt Instrument, Convertible, Stock Price Trigger | $ / shares				$ 22.92
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	(92,898)			(92,898)			(93,573)
Embedded derivative	173			173			214
Carrying value of liability component	57,275			57,275			56,641
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	341	314		675	622
Non-cash amortization of deferred financing costs	12	12		24	24
Non-cash change in value of derivative liability	(97)	6		(41)	8
Total interest expense related to the debentures	1,100	1,176		2,346	2,342
Convertible Senior Debentures, Due 2042 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 34,065			$ 34,065			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	91.4035
Debt effective conversion price | $ / shares	$ 10.94			$ 10.94
Debt Instrument, Convertible, Stock Price Trigger | $ / shares				$ 14.22
Convertible senior debentures call date	Jun. 07, 2022
Principal amount of repurchased debt	$ 69,060			$ 69,060
Liability and equity components of convertible debentures [Abstract]
Principal amount of the debentures	80,940			80,940			150,000
Unamortized discount	(46,919)			(46,919)			(87,600)
Embedded derivative	44			44			118
Carrying value of liability component	34,065			34,065			62,518
Equity component - net carrying value	31,229			31,229			57,874
Interest expense related to debentures [Abstract]
Contractual coupon interest	714	844		1,558	1,688
Non-cash amortization of debt discount	275	306		599	607
Non-cash amortization of deferred financing costs	11	14		24	27
Non-cash change in value of derivative liability	(26)	12		(15)	8
Total interest expense related to the debentures	974	1,176		2,166	2,330
Net carrying value of repurchased debt	29,037			29,037
Convertible Senior Notes, Due 2025 [Member]
Debt Instrument [Line Items]
Convertible Debt, Noncurrent	$ 488,518			$ 488,518			$ 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	(111,482)			(111,482)
Carrying value of liability component	488,518			488,518
Equity component - net carrying value	85,262			85,262
Interest expense related to debentures [Abstract]
Contractual coupon interest	713
Non-cash amortization of debt discount	556
Non-cash amortization of deferred financing costs	151
Total interest expense related to the debentures	1,420
Convertible senior debentures [Member]
Interest expense related to debentures [Abstract]
Contractual coupon interest	2,692	3,235		5,927	6,470
Non-cash amortization of debt discount	1,099	1,233		2,408	2,444
Non-cash amortization of deferred financing costs	39	48		86	95
Non-cash change in value of derivative liability	(156)	(1)		5	(28)
Total interest expense related to the debentures	$ 3,674	$ 4,515		$ 8,426	$ 8,981