Marketable Investment Securities (Details 4) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Fair value of marketable securities
Total debt securities	$ 1,201,133,000	$ 454,500,000
Equity securities	1,357,000	1,102,000
Corporate bonds
Fair value of marketable securities
Total debt securities	1,028,521,000	368,083,000
Other debt securities
Fair value of marketable securities
Total debt securities	172,612,000	86,417,000
Level 3
Fair value of marketable securities
Fair value of investments	0	0
Fair value measurements on recurring basis
Fair value of marketable securities
Total debt securities	1,201,133,000	454,500,000
Equity securities	1,357,000	1,102,000
Total marketable investment securities	1,202,490,000	455,602,000
Fair value measurements on recurring basis \| Corporate bonds
Fair value of marketable securities
Total debt securities	1,028,521,000	368,083,000
Fair value measurements on recurring basis \| Other debt securities
Fair value of marketable securities
Total debt securities	172,612,000	86,417,000
Fair value measurements on recurring basis \| Level 1
Fair value of marketable securities
Total debt securities	0	0
Equity securities	1,357,000	1,102,000
Total marketable investment securities	1,357,000	1,102,000
Fair value measurements on recurring basis \| Level 1 \| Corporate bonds
Fair value of marketable securities
Total debt securities	0	0
Fair value measurements on recurring basis \| Level 1 \| Other debt securities
Fair value of marketable securities
Total debt securities	0	0
Fair value measurements on recurring basis \| Level 2
Fair value of marketable securities
Total debt securities	1,201,133,000	454,500,000
Equity securities	0	0
Total marketable investment securities	1,201,133,000	454,500,000
Fair value measurements on recurring basis \| Level 2 \| Corporate bonds
Fair value of marketable securities
Total debt securities	1,028,521,000	368,083,000
Fair value measurements on recurring basis \| Level 2 \| Other debt securities
Fair value of marketable securities
Total debt securities	$ 172,612,000	$ 86,417,000

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Section 50
-Paragraph 2
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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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