Investments and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Marketable securities
Amortized Cost	$ 191,314	$ 97,016
Gross Unrealized Gains	95
Gross Unrealized Losses	(20)	(29)
Fair Value	191,389	96,987
Assets:
Investments	191,389	96,987
Transfers between Level 1, Level 2 or Level 3
Transfer from Level 1 to Level 2, assets	0	0
Transfer from Level 2 to Level 1, assets	0	0
Transfers into Level 3, assets	0	0
Transfer out of Level 3, assets	0	0
Transfer from Level 1 to Level 2, liabilities	0	0
Transfer from Level 2 to Level 1, liabilities	0	0
Transfers into Level 3, liabilities	0	0
Transfer out of Level 3, liabilities	0	0
U.S. treasury bills and notes
Marketable securities
Amortized Cost	189,816
Gross Unrealized Gains	91
Gross Unrealized Losses	(20)
Fair Value	189,887
Assets:
Investments	189,887
U.S. government agency bonds
Marketable securities
Amortized Cost	1,498	17,704
Gross Unrealized Gains	4
Gross Unrealized Losses		(3)
Fair Value	1,502	17,701
Assets:
Investments	1,502	17,701
U.S. treasury notes
Marketable securities
Amortized Cost		79,312
Gross Unrealized Losses		(26)
Fair Value		79,286
Assets:
Investments		79,286
Recurring
Assets:
Assets	283,287	404,051
Recurring \| Money market funds
Assets:
Cash equivalents	91,898	282,160
Recurring \| U.S. treasury bills and notes
Marketable securities
Fair Value	189,887
Assets:
Investments	189,887
Recurring \| U.S. treasury bills
Assets:
Cash equivalents		24,904
Recurring \| U.S. government agency bonds
Marketable securities
Fair Value	1,502	17,701
Assets:
Investments	1,502	17,701
Recurring \| U.S. treasury notes
Marketable securities
Fair Value		79,286
Assets:
Investments		79,286
Recurring \| Level 1
Assets:
Assets	91,898	282,160
Recurring \| Level 1 \| Money market funds
Assets:
Cash equivalents	91,898	282,160
Recurring \| Level 2
Assets:
Assets	191,389	121,891
Recurring \| Level 2 \| U.S. treasury bills and notes
Marketable securities
Fair Value	189,887
Assets:
Investments	189,887
Recurring \| Level 2 \| U.S. treasury bills
Assets:
Cash equivalents		24,904
Recurring \| Level 2 \| U.S. government agency bonds
Marketable securities
Fair Value	1,502	17,701
Assets:
Investments	$ 1,502	17,701
Recurring \| Level 2 \| U.S. treasury notes
Marketable securities
Fair Value		79,286
Assets:
Investments		$ 79,286

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