Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	$ 1,966	$ 1,071
Unrealized (Losses)	(556)	(5)
Debt Securities, Available-for-sale, Amortized Cost	433,319	417,332
Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	147,565	87,559
Fair Value	434,729	418,398
amortized cost [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	147,554	87,559
Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	11	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	183,308	211,089
Current Assets [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	1,163	246
Unrealized (Losses)	(84)	(4)
Current Assets [Member] | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	184,387	211,331
Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	792	825
Unrealized (Losses)	(472)	(1)
Debt Securities, Available-for-sale, Amortized Cost		118,684
Long-term investments | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	102,777	119,508
Long-term investments | Reported Value Measurement [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	102,457
Asset backed securities | Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	9	11
Unrealized (Losses)	0	0
Debt Securities, Available-for-sale, Amortized Cost	3,017	2,438
Asset backed securities | Current Assets [Member] | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	3,026	2,449
Asset backed securities | Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	79	193
Unrealized (Losses)	(72)	(1)
Debt Securities, Available-for-sale, Amortized Cost	39,983	40,001
Asset backed securities | Long-term investments | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	39,990	40,193
Corporate bonds | Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Corporate bonds | Cash and cash equivalents: | amortized cost [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	1,517
Corporate bonds | Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	17	51
Unrealized (Losses)	(84)	(1)
Debt Securities, Available-for-sale, Amortized Cost	23,975	33,048
Corporate bonds | Current Assets [Member] | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	23,908	33,098
Corporate bonds | Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	169	262
Unrealized (Losses)	(390)	0
Debt Securities, Available-for-sale, Amortized Cost	46,065	32,236
Corporate bonds | Long-term investments | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	45,844	32,498
US government agencies | Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	10
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
US government agencies | Cash and cash equivalents: | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	15,449
US government agencies | Cash and cash equivalents: | amortized cost [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	15,439
US government agencies | Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	1,137	184
Unrealized (Losses)	0	(3)
Debt Securities, Available-for-sale, Amortized Cost	155,316	173,364
US government agencies | Current Assets [Member] | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	156,453	173,545
US government agencies | Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	544	370
Unrealized (Losses)	(10)	0
Debt Securities, Available-for-sale, Amortized Cost	16,409	46,447
US government agencies | Long-term investments | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	16,943	46,817
Commercial Paper [Member] | Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Commercial Paper [Member] | Cash and cash equivalents: | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	1,200	3,399
Commercial Paper [Member] | Cash and cash equivalents: | amortized cost [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	1,200	3,399
Commercial Paper [Member] | Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	1,000	2,239
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Commercial Paper [Member] | Current Assets [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized (Losses)		0
Commercial Paper [Member] | Current Assets [Member] | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	1,000	2,239
Cash [Member] | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	94,365
Cash [Member] | Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	94,365	84,102
Money market funds | Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	35,033	58
Money market funds | Cash and cash equivalents: | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	35,033	$ 58
Corporate bonds | Cash and cash equivalents: | Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	$ 1,518