Cash, Cash Equivalents, And Marketable Securities Cash, Cash Equivalents, And Marketable Securities narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Cash, Cash Equivalents And Marketable Securities [Abstract]
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value	$ 1,750,611	$ 1,685,882
Debt securities gross unrealized gains	$ 300

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=73527792&loc=d3e27161-111563

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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

Cash, Cash Equivalents, And Marketable Securities [Abstract]

+ References

No definition available.

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