Fair Values (Details of Book Value and Fair Value of Financial Instruments) (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible due 2021 - Bond Hedge	$ 7,532,000	$ 1,028,000
Cash and cash equivalents	91,926,000	116,907,000	$ 150,618,000	$ 176,528,000
Convertible 2021 note hedge asset	10,684,000	2,062,000
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	91,926,000	116,907,000
Other investments	90,000	90,000
Installment receivables, net of reserves	1,555,000	1,796,000
Long-term debt (including current maturities of long-term debt)	(282,932,000)	(255,645,000)
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	91,926,000	116,907,000
Other investments	90,000	90,000
Installment receivables, net of reserves	1,555,000	1,796,000
Long-term debt (including current maturities of long-term debt)	(260,274,000)	(181,928,000)
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	10,684,000	2,062,000
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	10,684,000	2,062,000
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	7,532,000	1,028,000
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	7,532,000	1,028,000
Convertible Debt 2022 Conversion Feature [Domain] | Other Debt Obligations [Member] | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(11,295,000)	2,611,000
Convertible Debt 2022 Conversion Feature [Domain] | Other Debt Obligations [Member] | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(11,295,000)	2,611,000
Foreign currency forward contracts | Other Current Assets | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1,166,000	1,020,000
Foreign currency forward contracts | Other Current Assets | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1,166,000	1,020,000
Foreign currency forward contracts | Accrued Expenses | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(1,105,000)	(219,000)
Foreign currency forward contracts | Accrued Expenses | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(1,105,000)	(219,000)
Convertible Debt Conversion Feature [Member] | Other Debt Obligations [Member] | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(8,173,000)	1,458,000
Convertible Debt Conversion Feature [Member] | Other Debt Obligations [Member] | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	(8,173,000)	1,458,000
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	(11,295,000)	(2,611,000)
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	61,000	801,000
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	8,173,000	1,458,000
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	7,532,000	1,028,000
Convertible 2021 note hedge asset	10,684,000	2,062,000
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