Fair Values (Details of Book Value and Fair Value of Financial Instruments) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible due 2021 - Bond Hedge	$ 48,962,000	$ 46,915,000
Cash and cash equivalents	150,618,000	176,528,000	$ 76,836,000	$ 124,234,000
Convertible 2021 note hedge asset	48,919,000	46,680,000
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	150,618,000	176,528,000
Other investments	103,000	103,000
Installment receivables, net of reserves	1,698,000	1,809,000
Long-term debt (including current maturities of long-term debt)	(246,141,000)	(243,445,000)
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	150,618,000	176,528,000
Other investments	103,000	103,000
Installment receivables, net of reserves	1,698,000	1,809,000
Long-term debt (including current maturities of long-term debt)	(302,867,000)	(294,173,000)
Convertible Debt 2022 Note Hedge [Domain] | Other Long-Term Assets [Member] | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	48,919,000	46,680,000
Convertible Debt 2022 Note Hedge [Domain] | Other Long-Term Assets [Member] | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	48,919,000	46,680,000
Convertible Debt 2021 Note Hedge [Domain] | Other Long-Term Assets [Member] | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	48,962,000	46,915,000
Convertible Debt 2021 Note Hedge [Domain] | Other Long-Term Assets [Member] | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	48,962,000	46,915,000
Convertible Debt 2022 Conversion Feature [Domain] | Other Debt Obligations [Member] | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(55,224,000)	53,414,000
Convertible Debt 2022 Conversion Feature [Domain] | Other Debt Obligations [Member] | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(55,224,000)	53,414,000
Foreign currency forward contracts | Other Current Assets | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1,323,000	730,000
Foreign currency forward contracts | Other Current Assets | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1,323,000	730,000
Foreign currency forward contracts | Accrued Expenses | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(2,652,000)	(2,120,000)
Foreign currency forward contracts | Accrued Expenses | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(2,652,000)	(2,120,000)
Convertible Debt Conversion Feature [Member] | Other Debt Obligations [Member] | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(55,527,000)	53,154,000
Convertible Debt Conversion Feature [Member] | Other Debt Obligations [Member] | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(55,527,000)	53,154,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 [Member] | Foreign currency forward contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets (liabilities), at fair value, net	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 [Member] | Convertible Debt 2022 Conversion Feature [Domain]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets (liabilities), at fair value, net	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 [Member] | Convertible Debt Bond Hedge [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets (liabilities), at fair value, net	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Convertible Debt 2022 Conversion Feature [Domain]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible Debt Conversion Feature	(55,224,000)	(53,414,000)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Foreign currency forward contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets (liabilities), at fair value, net	(1,329,000)	(1,390,000)
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Convertible Debt Conversion Feature [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible 2021 debt conversion liability	55,527,000	53,154,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Convertible Debt Bond Hedge [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible due 2021 - Bond Hedge	48,962,000	46,915,000
Convertible 2021 note hedge asset	48,919,000	46,680,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Foreign currency forward contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets (liabilities), at fair value, net	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Convertible Debt 2022 Conversion Feature [Domain]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets (liabilities), at fair value, net	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Convertible Debt Bond Hedge [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets (liabilities), at fair value, net	$ 0	$ 0