Fair Value of Financial Instruments - Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [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	305,331	334,238
Commercial Paper [Member]
Debt Securities, Available-for-sale [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
US government agencies
Debt Securities, Available-for-sale [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
Corporate bonds
Debt Securities, Available-for-sale [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
Asset backed securities
Debt Securities, Available-for-sale [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