Marketable Investment Securities - Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair value of marketable securities
Debt securities	$ 1,870,818	$ 2,200,650
Equity securities	64,887	90,976
Total marketable investment securities	1,935,705	2,291,626
Corporate bonds
Fair value of marketable securities
Debt securities	1,470,693	1,735,653
Other debt securities
Fair value of marketable securities
Debt securities	400,125	464,997
Level 1
Fair value of marketable securities
Debt securities	10,597	9,474
Equity securities	58,400	85,298
Total marketable investment securities	68,997	94,772
Level 1 \| Corporate bonds
Fair value of marketable securities
Debt securities	0	0
Level 1 \| Other debt securities
Fair value of marketable securities
Debt securities	10,597	9,474
Level 2
Fair value of marketable securities
Debt securities	1,860,221	2,191,176
Equity securities	6,487	5,678
Total marketable investment securities	1,866,708	2,196,854
Level 2 \| Corporate bonds
Fair value of marketable securities
Debt securities	1,470,693	1,735,653
Level 2 \| Other debt securities
Fair value of marketable securities
Debt securities	$ 389,528	$ 455,523

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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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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Amount of investment in marketable security.

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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
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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