Amount 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3044-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Summary of Significant Accounting Policies - Concentrations (Details) $ in Thousands	Jun. 30, 2018 USD ($) issuer	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)
Concentration risk
Investments - debt securities	$ 14,202	$ 76,245
Investments - equity securities	38,830
Cash and cash equivalents	215,095	$ 192,083	$ 247,178
U.S. Treasury securities
Concentration risk
Investments - debt securities	9,900
Preferred stock
Concentration risk
Investments - equity securities	$ 38,800
Commercial paper
Concentration risk
Number of issuers \| issuer	12
Cash and cash equivalents	$ 165,600
Non-investment grade \| Corporate debt securities
Concentration risk
Investments - debt securities	$ 4,300
Number of issuers \| issuer	2
Non-investment grade \| Preferred stock
Concentration risk
Number of issuers \| issuer	5
Investment grade \| Preferred stock
Concentration risk
Number of issuers \| issuer	2