Long-Term Debt (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) $ / shares	Sep. 28, 2019 USD ($)	Jun. 29, 2019 USD ($)	Mar. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 29, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) d $ / shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instruments [Abstract]
Credit facility		$ 0				$ 0				$ 0	$ 0
Deferred financing costs		(16,784)				(14,968)				(16,784)	(14,968)
Long-term debt		499,147				494,509				499,147	494,509
Less current portion		0				0				0	0
Long-term debt, less current portion		499,147				494,509				499,147	494,509
Allocated liability component of repurchased debt	$ 133,647	9,568				108,059		$ 133,647		9,568	108,059
Allocated equity component of repurchased debt	451,344	18,295				267,945		451,344		18,295	267,945
Loss on early extinguishment of debt	17,309	723	$ 0	$ 0	$ 1,307	9,274	$ 0	17,309	$ 0	2,030	26,583	$ 0
Interest expense [Abstract]
Contractual coupon interest										13,998	16,090	12,938
Non-cash amortization of debt discount										14,146	10,769	4,984
Non-cash amortization of deferred financing costs										1,823	1,185	189
Non-cash change in value of derivative liability										(42)	128	(316)
Total interest expense related to the debentures										29,925	28,172	17,795
Committed and uncommitted short-term credit lines		6,000				7,000				6,000	7,000
Aggregate annual maturities of long-term debt [Abstract]
Interest paid										16,177	23,859	21,216
Convertible Debt [Member]
Debt Instruments [Abstract]
Loss on early extinguishment of debt											(9,274)
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt		617,190				636,556				617,190	636,556
Unamortized discount		(101,259)				(127,149)				(101,259)	(127,149)
Embedded derivative		0				70				0	70
Carrying value of liability component		515,931				509,477				515,931	509,477
Equity component (including temporary equity) - net carrying value		92,391				100,354				$ 92,391	100,354
Debt Instrument, Period to accrue contingent interest subsequent to debt issuance										10 years
Revolving Credit Facility [Member]
Line of Credit Facility [Abstract]
Line of credit facility, initiation date										Jun. 05, 2019
Line of credit facility, maximum borrowing capacity		750,000								$ 750,000
Line of credit facility, expiration date										Jun. 05, 2024
Incremental revolving commitments										$ 300,000
Basis spread on variable rate										1.50%
Commitment fees										0.25%
Line of credit facility, decrease in commitment fee										(0.05%)
Investments pro forma ratio										2.75
Restricted payments pro forma ratio										2.50
Restricted payments, annual limit		100,000								$ 100,000
Restricted payments ,total limit		300,000								300,000
Restricted payments ,rollover limit		25,000								25,000
Restricted payments, unused amount										100,000
Event of default - judgment limit		50,000								50,000
Available borrowing capacity		747,912								747,912
Letters of credit outstanding		2,088								2,088
Previous Credit Facility [Member]
Line of Credit Facility [Abstract]
Line of credit facility, maximum borrowing capacity		640,000								$ 640,000
Line of credit facility, expiration date										Dec. 10, 2020
Available borrowing capacity						636,211					636,211
Letters of credit outstanding						3,789					3,789
Convertible Senior Notes, Due 2025 [Member]
Debt Instruments [Abstract]
Convertible debt		$ 509,128				$ 495,203				$ 509,128	$ 495,203
Issuance date										Jun. 12, 2018
Maturity date										Jun. 15, 2025
Cash coupon rate		2.25%				2.25%				2.25%	2.25%
Nonconvertible debt borrowing rate at issuance		5.50%								5.50%
Effective conversion rate										31.8083
Effective conversion price (in dollars per share) | $ / shares		$ 31.44								$ 31.44
130% of the conversion price (in dollars per share) | $ / shares										$ 40.87
Debt instrument percentage of conversion price										130.00%
Debt instrument, percentage of sale price of common stock		98.00%								98.00%
Percentage of redemption price to principal amount										100.00%
Threshold percentage of stock price trigger										150.00%
Trading period to trigger conversion, minimum | d										20
Consecutive trading period prior to notice of redemption | d										30
Maximum threshold of quarterly cash dividends per share of common stock for not adjusting conversion rate of convertible notes (in dollars per share) | $ / shares										$ 0.085
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt		$ 600,000				$ 600,000				$ 600,000	$ 600,000
Interest expense [Abstract]
Contractual coupon interest										13,500	7,463
Non-cash amortization of debt discount										13,925	7,240
Non-cash amortization of deferred financing costs										1,816	1,059
Non-cash change in value of derivative liability										0	0
Total interest expense related to the debentures										29,241	15,762
Convertible Senior Notes, Due 2025 [Member] | Convertible Debt [Member]
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt		600,000				600,000				600,000	600,000
Unamortized discount		(90,872)				(104,797)				(90,872)	(104,797)
Embedded derivative		0				0				0	0
Carrying value of liability component		509,128				495,203				509,128	495,203
Equity component (including temporary equity) - net carrying value		85,262				85,262				85,262	85,262
Convertible Senior Debentures, Due 2040 [Member]
Debt Instruments [Abstract]
Convertible debt		$ 126				539				$ 126	539
Issuance date										Nov. 09, 2010
Maturity date										Nov. 15, 2040
Cash coupon rate		2.25%								2.25%
Nonconvertible debt borrowing rate at issuance		8.00%								8.00%
Effective conversion rate										80.0018
Effective conversion price (in dollars per share) | $ / shares		$ 12.50								$ 12.50
130% of the conversion price (in dollars per share) | $ / shares										$ 16.25
Call date										Nov. 20, 2020
Conversion period before maturity date										3 months
Debt instrument percentage of conversion price										130.00%
Debt instrument, percentage of sale price of common stock		98.00%								98.00%
Percentage of redemption price to principal amount										100.00%
Threshold percentage of stock price trigger										150.00%
Trading period to trigger conversion, minimum | d										20
Consecutive trading period prior to notice of redemption | d										30
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt		$ 300								$ 300
Interest expense [Abstract]
Principal amount of repurchased debt	220,000	1,010				53,690		220,000		1,010	53,690
Net carrying value of repurchased debt	89,276	417				22,018		89,276		417	22,018
Convertible Senior Debentures, Due 2041 [Member]
Debt Instruments [Abstract]
Convertible debt		$ 6,677				12,812				$ 6,677	12,812
Issuance date										May 13, 2011
Maturity date										May 15, 2041
Cash coupon rate		2.25%								2.25%
Nonconvertible debt borrowing rate at issuance		8.375%								8.375%
Effective conversion rate										58.3812
Effective conversion price (in dollars per share) | $ / shares		$ 17.13								$ 17.13
130% of the conversion price (in dollars per share) | $ / shares										$ 22.27
Call date										May 20, 2021
Conversion period before maturity date										3 months
Debt instrument percentage of conversion price										130.00%
Debt instrument, percentage of sale price of common stock		98.00%								98.00%
Percentage of redemption price to principal amount										100.00%
Threshold percentage of stock price trigger										150.00%
Trading period to trigger conversion, minimum | d										20
Consecutive trading period prior to notice of redemption | d										30
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt		$ 16,890								$ 16,890
Interest expense [Abstract]
Principal amount of repurchased debt		16,188				116,922				16,188	116,922
Net carrying value of repurchased debt		6,282				45,157				6,282	45,157
Convertible Senior Debentures 2042 [Member]
Debt Instruments [Abstract]
Convertible debt		0				923				0	923
Interest expense [Abstract]
Principal amount of repurchased debt	69,060	2,168				78,772		69,060		2,168	78,772
Net carrying value of repurchased debt	29,037	924				33,466		29,037		924	33,466
Convertible Senior Debentures [Member]
Debt Instruments [Abstract]
Purchase price of extinguished debt	$ 584,991	27,863				376,004		$ 584,991		27,863	376,004
Loss on early extinguishment of debt											26,583
Interest expense [Abstract]
Contractual coupon interest										498	8,627	12,938
Non-cash amortization of debt discount										221	3,529	4,984
Non-cash amortization of deferred financing costs										7	126	189
Non-cash change in value of derivative liability										(42)	128	(316)
Total interest expense related to the debentures										684	12,410	$ 17,795
Principal amount of repurchased debt		19,366				538,444				19,366	538,444
Convertible Senior Debentures [Member] | Convertible Debt [Member]
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt		17,190				36,556				17,190	36,556
Unamortized discount		(10,387)				(22,352)				(10,387)	(22,352)
Embedded derivative		0				70				0	70
Carrying value of liability component		6,803				14,274				6,803	14,274
Equity component (including temporary equity) - net carrying value		$ 7,129				$ 15,092				$ 7,129	$ 15,092