General - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Investments	$ 155,518	$ 143,459
Cash held in bank accounts	38,900	464,900
U.S. Government and agency securities
Assets
Investments	16,530	42,399
Corporate debt securities
Assets
Investments	125,389	99,749
Taxable municipal bonds
Assets
Investments	12,299
Certificates of deposit
Assets
Investments	1,300	1,311
Level 1 \| 4.400% Senior Notes due 2024
Liabilities
Long-term Debt, Fair Value	395,600
Level 2 \| 2017 Credit Agreement, senior unsecured term loan facility
Liabilities
Long-term Debt, Fair Value	150,000
Fair Value, Recurring
Assets
Total assets held at fair value	1,155,038	823,013
Fair Value, Recurring \| Other Than Cash In Entity Bank Accounts Member
Assets
Cash and cash equivalents	999,520	679,554
Fair Value, Recurring \| U.S. Government and agency securities
Assets
Investments	16,530	42,399
Fair Value, Recurring \| Corporate debt securities
Assets
Investments	125,389	99,749
Fair Value, Recurring \| Taxable municipal bonds
Assets
Investments	12,299
Fair Value, Recurring \| Certificates of deposit
Assets
Investments	1,300	1,311
Fair Value, Recurring \| Level 1
Assets
Total assets held at fair value	16,530	42,399
Fair Value, Recurring \| Level 1 \| U.S. Government and agency securities
Assets
Investments	16,530	42,399
Fair Value, Recurring \| Level 2
Assets
Total assets held at fair value	1,138,508	780,614
Fair Value, Recurring \| Level 2 \| Other Than Cash In Entity Bank Accounts Member
Assets
Cash and cash equivalents	999,520	679,554
Fair Value, Recurring \| Level 2 \| Corporate debt securities
Assets
Investments	125,389	99,749
Fair Value, Recurring \| Level 2 \| Taxable municipal bonds
Assets
Investments	12,299
Fair Value, Recurring \| Level 2 \| Certificates of deposit
Assets
Investments	$ 1,300	$ 1,311

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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
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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 942
-SubTopic 210
-Section S99
-Paragraph 1
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-Name Accounting Standards Codification
-Topic 326
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-Section 45
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-Topic 320
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-Section 45
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-Topic 320
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-Section 50
-Paragraph 2
-Subparagraph (aa)
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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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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.

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