Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Changes in Level 3 Instruments (Details)	Mar. 31, 2018 USD ($)
Fair values transfers between Level 1 and Level 2
Fair value assets transfer from level 1 to level 2	$ 0
Fair value liabilities transfer from level 1 to level 2	0
Fair value assets transfer from level 2 to level 1	0
Fair value liabilities transfer from level 2 to level 1	$ 0

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

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- Definition

Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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- Definition

Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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