Investments and Fair Value Measurements (Quantitative Information about Level 3 Fair Value Measurements) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt	$ 40,785		$ 76,413
Significant Unobservable Inputs (Level 3) \| Discounted cash flow
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value of derivatives embedded within convertible debt	$ 65,846		$ 76,413
Assumed annual stock dividend	5.00%	5.00%
Assumed annual cash dividend (in dollars per share)	$ 1.60	$ 1.60
Stock price (in dollars per share)	$ 20.39		$ 22.38
Convertible trading price (as a percentage of par value)	109.71%	115.19%
Volatility	17.51%	17.98%
Implied credit spread	4.50%	3.50%
Significant Unobservable Inputs (Level 3) \| Discounted cash flow \| Minimum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Implied credit spread	4.00%	3.00%
Significant Unobservable Inputs (Level 3) \| Discounted cash flow \| Maximum
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Implied credit spread	5.00%	4.00%

Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as a liability.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of dividends expected to be paid to holders of the underlying shares or contracted instruments over the term of the instruments life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Expected dividends to be paid to holders of the underlying shares or financial instruments (expressed as a percentage of the share or instrument's price).

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- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

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Percentage of adjustment to the observed price for a similar asset or liability when an identical transaction does not exist, used as an input to measure fair value.

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Risk associated with risk of default, credit spread risk and downgrade risk, expressed as a percentage, used as an input to measure fair value.

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- Definition

Price of a single share of a number of saleable stocks of a company.

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