Long-Term Debt (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) d $ / shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Instruments [Abstract]
Credit facility	$ 0	$ 0
Deferred financing costs	(11,512)	(16,784)
Long-term debt	394,886	499,147
Less current portion	0	0
Long-term debt, less current portion	394,886	499,147
Purchase price of extinguished debt			$ 960,995
Allocated liability component of repurchased debt		9,568	241,706
Allocated equity component of repurchased debt		18,295	719,289
Loss on early extinguishment of debt	8,073	2,030	26,583
Interest expense [Abstract]
Contractual coupon interest	12,185	13,998	16,090
Non-cash amortization of debt discount	13,161	14,146	10,769
Other Noncash Interest Expense (Income)	1,623	1,781	1,313
Total interest expense related to the debentures	26,969	29,925	28,172
Principal amount of repurchased debt	151,546	19,366	538,444
Committed and uncommitted short-term credit lines	6,000	6,000
Aggregate annual maturities of long-term debt [Abstract]
Interest paid	15,450	16,177	23,859
Convertible Debt [Member]
Debt Instruments [Abstract]
Loss on early extinguishment of debt			26,583
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt	465,644	617,190
Unamortized discount	(59,246)	(101,259)
Carrying value of liability component	406,398	515,931
Equity component (including temporary equity) - net carrying value	$ 66,248	92,391
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
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%
Investments pro forma ratio	2.75
Restricted payments pro forma ratio	2.50
Restricted payments, annual limit	$ 100,000
Restricted payments ,total limit	300,000
Restricted payments ,rollover limit	25,000
Restricted payments, unused amount	100,000
Event of default - judgment limit	50,000
Available borrowing capacity	748,690	747,912
Letters of credit outstanding	1,310	2,088
Previous Credit Facility [Member]
Line of Credit Facility [Abstract]
Line of credit facility, maximum borrowing capacity	640,000
Convertible Senior Notes, Due 2025 [Member]
Debt Instruments [Abstract]
Convertible debt	$ 406,268	509,128
Issuance date	Jun. 12, 2018
Maturity date	Jun. 15, 2025
Cash coupon rate	2.25%
Nonconvertible debt borrowing rate at issuance	5.50%
Effective conversion rate	31.8836
Effective conversion price (in dollars per share) | $ / shares	$ 31.36
130% of the conversion price (in dollars per share) | $ / shares	$ 40.77
Debt instrument percentage of conversion price	130.00%
Debt instrument, percentage of sale price of common stock	98.00%
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
Purchase price of extinguished debt	$ 128,328
Allocated liability component of repurchased debt	118,587
Allocated equity component of repurchased debt	9,741
Loss on early extinguishment of debt	4,600
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt	465,344	600,000
Unamortized discount	(59,076)	(90,872)
Carrying value of liability component	406,268	509,128
Equity component (including temporary equity) - net carrying value	66,127	85,262
Interest expense [Abstract]
Contractual coupon interest	12,097	13,500	7,463
Non-cash amortization of debt discount	13,118	13,925	7,240
Other Noncash Interest Expense (Income)	1,623	1,816	1,059
Total interest expense related to the debentures	26,838	29,241	15,762
Principal amount of repurchased debt	134,656
Net carrying value of repurchased debt	115,978
Convertible Senior Debentures, Due 2040 [Member]
Debt Instruments [Abstract]
Convertible debt	$ 130	126
Issuance date	Nov. 09, 2010
Maturity date	Nov. 15, 2040
Cash coupon rate	2.25%
Nonconvertible debt borrowing rate at issuance	8.00%
Effective conversion rate	81.8143
Effective conversion price (in dollars per share) | $ / shares	$ 12.22
130% of the conversion price (in dollars per share) | $ / shares	$ 15.89
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%
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
Debt Instrument, Period to accrue contingent interest subsequent to debt issuance	10 years
Interest expense [Abstract]
Principal amount of repurchased debt		1,010	273,690
Net carrying value of repurchased debt		417	111,294
Convertible Senior Debentures, Due 2041 [Member]
Debt Instruments [Abstract]
Convertible debt	$ 0	6,677
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%
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
Purchase price of extinguished debt	$ 23,355
Allocated liability component of repurchased debt	10,075
Allocated equity component of repurchased debt	13,280
Loss on early extinguishment of debt	3,473
Interest expense [Abstract]
Principal amount of repurchased debt	16,890	16,188	116,922
Net carrying value of repurchased debt	6,715	6,282	45,157
Convertible Senior Debentures 2042 [Member]
Interest expense [Abstract]
Principal amount of repurchased debt		2,168	147,832
Net carrying value of repurchased debt		924	62,503
Convertible Senior Debentures [Member]
Debt Instruments [Abstract]
Purchase price of extinguished debt		27,863
Interest expense [Abstract]
Contractual coupon interest	88	498	8,627
Non-cash amortization of debt discount	43	221	3,529
Other Noncash Interest Expense (Income)	0	(35)	254
Total interest expense related to the debentures	131	684	$ 12,410
Convertible Senior Debentures [Member] | Convertible Debt [Member]
Liability and equity components of convertible debentures [Abstract]
Principal amount of the convertible debt	300	17,190
Unamortized discount	(170)	(10,387)
Carrying value of liability component	130	6,803
Equity component (including temporary equity) - net carrying value	$ 121	$ 7,129