Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair value of marketable securities
Debt securities	$ 72,554	$ 164,160
Equity Securities	71,943	93,367
Total marketable investment securities	144,497	257,527
Transfer of investments from Level 1 to Level 2	0
Transfer of investments from Level 2 to Level 1	0
U.S. Treasury and agency securities
Fair value of marketable securities
Debt securities	72,301	74,788
Commercial paper
Fair value of marketable securities
Debt securities		24,407
Corporate securities
Fair value of marketable securities
Debt securities		63,809
Other (including restricted)
Fair value of marketable securities
Debt securities	253	1,156
Fair value measurements on recurring basis
Fair value of marketable securities
Cash equivalents (including restricted)		315,030
Equity Securities	71,943	93,367
Total marketable investment securities	144,497	257,527
Fair value measurements on recurring basis \| U.S. Treasury and agency securities
Fair value of marketable securities
Debt securities	72,301	74,788
Fair value measurements on recurring basis \| Commercial paper
Fair value of marketable securities
Debt securities		24,407
Fair value measurements on recurring basis \| Corporate securities
Fair value of marketable securities
Debt securities		63,809
Fair value measurements on recurring basis \| Other (including restricted)
Fair value of marketable securities
Debt securities	253	1,156
Fair value measurements on recurring basis \| Level 1
Fair value of marketable securities
Cash equivalents (including restricted)		140
Equity Securities	71,943	93,367
Total marketable investment securities	144,244	168,155
Fair value measurements on recurring basis \| Level 1 \| U.S. Treasury and agency securities
Fair value of marketable securities
Debt securities	72,301	74,788
Fair value measurements on recurring basis \| Level 2
Fair value of marketable securities
Cash equivalents (including restricted)		314,890
Total marketable investment securities	253	89,372
Fair value measurements on recurring basis \| Level 2 \| Commercial paper
Fair value of marketable securities
Debt securities		24,407
Fair value measurements on recurring basis \| Level 2 \| Corporate securities
Fair value of marketable securities
Debt securities		63,809
Fair value measurements on recurring basis \| Level 2 \| Other (including restricted)
Fair value of marketable securities
Debt securities	$ 253	$ 1,156

Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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