General - Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Investments	$ 356,481	$ 345,157
Cash held in bank accounts	194,400	11,700
U.S. Government and agency securities
Assets
Investments	90,713	104,159
Corporate debt securities
Assets
Investments	264,463	239,693
Certificates of deposit
Assets
Investments	1,305	1,305
Level 1 \| 4.400% Senior Notes due 2024
Liabilities
Long-term Debt, Fair Value	331,200
Level 2 \| 2017 Credit Agreement, senior unsecured term loan facility
Liabilities
Long-term Debt, Fair Value	276,300
Fair Value, Recurring
Assets
Total assets held at fair value	546,037	658,666
Fair Value, Recurring \| Other Than Cash In Entity Bank Accounts Member
Assets
Cash and cash equivalents	189,556	313,509
Fair Value, Recurring \| U.S. Government and agency securities
Assets
Investments	90,714	104,159
Fair Value, Recurring \| Corporate debt securities
Assets
Investments	264,462	239,693
Fair Value, Recurring \| Certificates of deposit
Assets
Investments	1,305	1,305
Fair Value, Recurring \| Level 1
Assets
Total assets held at fair value	90,714	104,159
Fair Value, Recurring \| Level 1 \| U.S. Government and agency securities
Assets
Investments	90,714	104,159
Fair Value, Recurring \| Level 2
Assets
Total assets held at fair value	455,323	554,507
Fair Value, Recurring \| Level 2 \| Other Than Cash In Entity Bank Accounts Member
Assets
Cash and cash equivalents	189,556	313,509
Fair Value, Recurring \| Level 2 \| Corporate debt securities
Assets
Investments	264,462	239,693
Fair Value, Recurring \| Level 2 \| Certificates of deposit
Assets
Investments	$ 1,305	$ 1,305

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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- Definition

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.

+ References

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

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