Fair Value of Financial Instruments - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 226,570	$ 214,731
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	55,843	85,617
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	22,918	33,890
Debt Securities, Available-for-sale, Amortized Cost	433,319	417,332
Debt Securities, Available-for-sale	305,331	334,238
Level 1 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money Market Funds, at Carrying Value	35,033	58
Debt Securities, Available-for-sale		58
Financial Instruments, Owned, at Fair Value	35,033
Level 2 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money Market Funds, at Carrying Value	0	0
Financial Instruments, Owned, at Fair Value	305,331	334,238
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	2,200	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
Debt Securities, Available-for-sale	2,200	5,638
Commercial paper | Level 1 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial Paper, at Carrying Value	0	0
Commercial paper | Level 2 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial Paper, at Carrying Value	2,200	5,638
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	171,901	173,546
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	15,712	46,816
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	1,231	0
Debt Securities, Available-for-sale	188,844	220,362
US government agencies | Level 1 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
US Government Agencies Securities, at Carrying Value	0	0
US government agencies | Level 2 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
US Government Agencies Securities, at Carrying Value	188,844	220,362
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	25,428	33,098
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	24,156	23,251
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	21,687	9,247
Debt Securities, Available-for-sale	71,271	65,596
Corporate bonds | Level 1 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Corporate Debt, at Fair Value	0	0
Corporate bonds | Level 2 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Corporate Debt, at Fair Value	71,271	65,596
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	27,041	2,449
Available-for-sale Securities, Debt and Equity Maturities, Year One through Two, Fair Value	15,975	15,550
Available-for-sale Securities, Debt and Equity Maturities, After Year Two, Fair Value	0	24,643
Debt Securities, Available-for-sale	43,016	42,642
Asset backed securities | Level 1 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset-Backed Securities, at Carrying Value	0	0
Asset backed securities | Level 2 | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset-Backed Securities, at Carrying Value	43,016	42,642
Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money Market Funds, at Carrying Value	35,033	58
Financial Instruments, Owned, at Fair Value	340,364	334,296
Fair Value | Commercial paper | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commercial Paper, at Carrying Value	2,200	5,638
Fair Value | US government agencies | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
US Government Agencies Securities, at Carrying Value	188,844	220,362
Fair Value | Corporate bonds | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, Corporate Debt, at Fair Value	71,271	65,596
Fair Value | Asset backed securities | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset-Backed Securities, at Carrying Value	43,016	42,642
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	11	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
Cash and cash equivalents: | US government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	10
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Cash and cash equivalents: | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 0	$ 0