General - Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets
Investments	$ 394,519	$ 385,743
Cash held in bank accounts	4,300	8,800
Liabilities
Contingent consideration	119	10,124
U.S. Government and agency securities
Assets
Investments	110,361	67,815
Obligations of government-sponsored enterprises
Assets
Investments	1,501	5,229
Corporate debt securities
Assets
Investments	257,507	292,049
Certificates of deposit
Assets
Investments	25,150	20,650
Level 1 \| 4.400% Senior Notes due 2024
Liabilities
Fair value of debt	399,000
Level 2 \| 2017 Credit Agreement, senior unsecured term loan facility
Liabilities
Fair value of debt	319,400
Fair value measured on recurring basis
Assets
Total assets held at fair value	650,602	649,205
Liabilities
Contingent consideration	119	10,124
Total liabilities held at fair value	119	10,124
Fair value measured on recurring basis \| Other than cash held in bank accounts by Company
Assets
Cash and cash equivalents	256,083	263,462
Fair value measured on recurring basis \| U.S. Government and agency securities
Assets
Investments	110,361	67,815
Fair value measured on recurring basis \| Obligations of government-sponsored enterprises
Assets
Investments	1,501	5,229
Fair value measured on recurring basis \| Corporate debt securities
Assets
Investments	257,507	292,049
Fair value measured on recurring basis \| Certificates of deposit
Assets
Investments	25,150	20,650
Fair value measured on recurring basis \| Level 1
Assets
Total assets held at fair value	110,361	67,815
Fair value measured on recurring basis \| Level 1 \| U.S. Government and agency securities
Assets
Investments	110,361	67,815
Fair value measured on recurring basis \| Level 2
Assets
Total assets held at fair value	540,241	581,390
Fair value measured on recurring basis \| Level 2 \| Other than cash held in bank accounts by Company
Assets
Cash and cash equivalents	256,083	263,462
Fair value measured on recurring basis \| Level 2 \| Obligations of government-sponsored enterprises
Assets
Investments	1,501	5,229
Fair value measured on recurring basis \| Level 2 \| Corporate debt securities
Assets
Investments	257,507	292,049
Fair value measured on recurring basis \| Level 2 \| Certificates of deposit
Assets
Investments	25,150	20,650
Fair value measured on recurring basis \| Level 3
Liabilities
Contingent consideration	119	10,124
Total liabilities held at fair value	$ 119	$ 10,124

Fair value of contingent consideration in a business combination as of the balance sheet date.

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

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

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

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Fair value of financial and nonfinancial obligations.

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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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