Financial Instruments Financial Instruments narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Convertible Debt, Fair Value Disclosures	$ 4,300,000
Debt securities gross unrealized gains	100
Debt instrument fair value senior notes 2023	751,900
Debt instrument fair value senior notes 2025	509,300
Debt instrument fair value senior notes 2027	527,300
Fair Value, Inputs, Level 1 [Member]
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	$ 1,277,071	$ 2,169,197

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

+ References

No definition available.

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- Definition

The fair value as of the balance sheet date of firm holdings in debt obligations issued by the US government including short-term Treasury bills, medium-term Treasury notes, and long-term Treasury bonds, as well as debt issued by agencies. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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- Definition

Debt instrument fair value senior notes 2023

+ References

No definition available.

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- Definition

Debt instrument fair value senior notes 2025

+ References

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- Definition

Senior Notes 2027

+ References

No definition available.

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