Fair Value Measurements (Details Textual) - USD ($) $ in Thousands	3 Months Ended
Jun. 30, 2018	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Noncurrent	$ 91,700	$ 97,896
Cost Method Investments, Fair Value Disclosure	49,600
Two And One Two Five Percent 2019 Senior Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Face Amount	$ 500,000
Debt Instrument, Interest Rate, Stated Percentage	2.125%
Debt Instrument, Maturity Date	Mar. 15, 2019
Fair value of convertible notes	$ 498,000
Three Point Zero Percent 2021 Senior Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Face Amount	$ 500,000
Debt Instrument, Interest Rate, Stated Percentage	3.00%
Debt Instrument, Maturity Date	Mar. 15, 2021
Fair value of convertible notes	$ 495,900
Two Point and Nine Five Percent 2024 Senior Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Face Amount	$ 750,000
Debt Instrument, Interest Rate, Stated Percentage	2.95%
Debt Instrument, Maturity Date	Jun. 01, 2024
Fair value of convertible notes	$ 713,400
Three Point Zero Percent 2021 Senior Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Face Amount	500,000	500,000
Two Point and Nine Five Percent 2024 Senior Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Face Amount	$ 750,000	750,000
Debt Instrument, Interest Rate, Stated Percentage	2.95%
Two And One Two Five Percent 2019 Senior Debentures [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Face Amount	$ 500,000	$ 500,000

Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
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Fair value portion of investments accounted under the cost method.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 325
-SubTopic 20
-Section 25
-Paragraph 1
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Face (par) amount of debt instrument at time of issuance.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Contractual interest rate for funds borrowed, under the debt agreement.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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