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Amount of interest expense incurred on long-term debt classified as other, including, but not limited to, interest on long-term notes and amortization of issuance costs.

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Convertible Notes (Interest Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 17, 2017	Aug. 16, 2013
Interest expense related to notes
Amortization of Debt Issuance Costs and Discounts	$ 7,578	$ 6,749	$ 6,372
Additional paid in capital	$ 1,212,798	$ 1,181,230
Convertible Senior Notes [Member] \| Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.375%			1.375%
Convertible Senior Notes [Member] \| 1.125% Convertible Senior Notes due 2018
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.125%	1.125%			1.125%
Convertible notes
Interest expense related to notes
Total interest expense on convertible notes	$ 9,356	$ 8,302	7,939
Additional paid in capital	111,300	93,400
Convertible notes \| Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]
Interest expense related to notes
Coupon interest	290
Amortization of Debt Issuance Costs and Discounts	$ 768
Debt Instrument, Interest Rate, Effective Percentage	4.90%
Debt Instrument, Face Amount	$ 172,500	0		$ 172,500
Convertible notes \| 1.125% Convertible Senior Notes due 2018
Interest expense related to notes
Coupon interest	1,488	1,553	1,567
Amortization of Debt Issuance Costs and Discounts	$ 6,810	$ 6,749	$ 6,372
Debt Instrument, Interest Rate, Effective Percentage	5.50%	5.50%
Debt Instrument, Face Amount	$ 81,207	$ 138,000			$ 138,000