Fair Value of Financial Instruments - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	$ 235,921	$ 214,731
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	57,764	85,617
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	39,719	33,890
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money Market Funds, at Carrying Value	5,638	58
Commercial Paper, at Carrying Value	3,848	5,638
US Government Agencies Securities, at Carrying Value	215,982	220,362
Asset-Backed Securities, at Carrying Value	42,411	42,642
Financial Instruments, Owned, Corporate Debt, at Fair Value	70,124	65,596
Financial Instruments, Owned, at Fair Value	339,042	334,296
Financial Instruments, Owned, Other Sovereign Government Obligations, at Fair Value	1,039
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money Market Funds, at Carrying Value	5,638	58
Commercial Paper, at Carrying Value	0	0
US Government Agencies Securities, at Carrying Value	0	0
Asset-Backed Securities, at Carrying Value	0	0
Financial Instruments, Owned, Corporate Debt, at Fair Value	0	0
Financial Instruments, Owned, at Fair Value	5,638	58
Financial Instruments, Owned, Other Sovereign Government Obligations, at Fair Value	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money Market Funds, at Carrying Value	0	0
Commercial Paper, at Carrying Value	3,848	5,638
US Government Agencies Securities, at Carrying Value	215,982	220,362
Asset-Backed Securities, at Carrying Value	42,411	42,642
Financial Instruments, Owned, Corporate Debt, at Fair Value	70,124	65,596
Financial Instruments, Owned, at Fair Value	333,404	334,238
Financial Instruments, Owned, Other Sovereign Government Obligations, at Fair Value	1,039
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	3,848	5,638
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	0	0
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	0	0
US government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	200,994	173,546
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	14,988	46,816
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	0	0
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	24,516	33,098
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	24,506	23,251
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	21,102	9,247
Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value	6,563	2,449
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	18,270	15,550
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	17,578	24,643
Cash and cash equivalents:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Cash and cash equivalents: \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 0	$ 0

Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value

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- Definition

Available-for-sale Securities, Debt and Equity Maturities, Next Twelve Months, Fair Value

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- Definition

Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value

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Investments in notes or bonds that are collateralized by a specific group of underlying assets owned by the issuer of the notes or bonds which are 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. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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- Definition

Unsecured short-term debt instrument issued by corporations which are 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. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The aggregate fair value as of the balance sheet date of financial instruments and other positions owned by the entity including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities. Includes both pledged and unpledged holdings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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Fair value of firm holdings in corporate fixed income securities. Includes pledged and unpledged holdings.

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The fair value as of the balance sheet date of firm holdings in fixed income obligations issued by foreign governments. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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Debt securities issued by a United States government agency such as Government National Mortgage Association (Ginnie Mae), Federal Home Loan Mortgage Corporation (Freddie Mac), and Federal National Mortgage Association (Fannie Mae), which are 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. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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