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Convertible Notes (Interest Expense) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	9 Months Ended			12 Months Ended
	Mar. 31, 2020	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 17, 2017	Aug. 16, 2013
Interest expense related to notes
Amortization of Debt Issuance Costs and Discounts	$ 1,773	$ 3,571	$ 5,394	$ 5,104	$ 7,587	$ 7,243	$ 6,854	$ 9,243
Additional paid in capital	$ 1,264,000	$ 1,274,136	$ 1,280,051	$ 1,254,344		$ 1,270,426	$ 1,261,142
Convertible senior notes | 1.375% convertible notes due 2023
Debt instrument
Debt Instrument, Interest Rate, Stated Percentage						1.375%	1.375%	1.375%	1.375%
Convertible senior notes | 1.125% Convertible Senior Notes due 2018
Debt instrument
Debt Instrument, Interest Rate, Stated Percentage								1.125%		1.125%
Convertible senior notes
Interest expense related to notes
Total interest expense on convertible notes						$ 9,615	$ 9,226	$ 11,991
Convertible senior notes | 1.375% convertible notes due 2023
Interest expense related to notes
Coupon interest						2,372	2,372	2,372
Amortization of Debt Issuance Costs and Discounts						$ 7,243	$ 6,854	$ 6,486
Debt Instrument, Interest Rate, Effective Percentage						4.90%	4.90%	4.90%
Debt Instrument, Face Amount						$ 172,500	$ 172,500		$ 172,500
Convertible senior notes | 1.125% Convertible Senior Notes due 2018
Interest expense related to notes
Coupon interest						0	0	$ 377
Amortization of Debt Issuance Costs and Discounts						$ 0	$ 0	$ 2,756
Debt Instrument, Interest Rate, Effective Percentage								5.50%
Debt Instrument, Face Amount										$ 138,000

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Value received from shareholders in common stock-related transactions that are in excess of par value or stated value and amounts received from other stock-related transactions. Includes only common stock transactions (excludes preferred stock transactions). May be called contributed capital, capital in excess of par, capital surplus, or paid-in capital.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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