Investments and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Marketable securities
Amortized Cost	$ 214,912	$ 97,016
Gross Unrealized Gains	174
Gross Unrealized Losses	(8)	(29)
Fair Value	215,078	96,987
Assets:
Investments	215,078	96,987
Transfers between Level 1, Level 2 or Level 3
Transfer from Level 1 to Level 2, assets	0
Transfer from Level 2 to Level 1, assets	0
Transfers into Level 3, assets	0
Transfer out of Level 3, assets	0
Transfer from Level 1 to Level 2, liabilities	0
Transfer from Level 2 to Level 1, liabilities	0
Transfers into Level 3, liabilities	0
Transfer out of Level 3, liabilities	0
U.S. treasury bills and notes
Marketable securities
Amortized Cost	201,045
Gross Unrealized Gains	165
Gross Unrealized Losses	(8)
Fair Value	201,202
Assets:
Investments	201,202
U.S. government agency bonds
Marketable securities
Amortized Cost	13,867	17,704
Gross Unrealized Gains	9
Gross Unrealized Losses		(3)
Fair Value	13,876	17,701
Assets:
Investments	13,876	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	324,689	404,051
Recurring \| Money market funds
Assets:
Cash equivalents	109,611	282,160
Recurring \| U.S. treasury bills and notes
Marketable securities
Fair Value	201,202
Assets:
Investments	201,202
Recurring \| U.S. treasury bills
Assets:
Cash equivalents		24,904
Recurring \| U.S. government agency bonds
Marketable securities
Fair Value	13,876	17,701
Assets:
Investments	13,876	17,701
Recurring \| U.S. treasury notes
Marketable securities
Fair Value		79,286
Assets:
Investments		79,286
Recurring \| Level 1
Assets:
Assets	109,611	282,160
Recurring \| Level 1 \| Money market funds
Assets:
Cash equivalents	109,611	282,160
Recurring \| Level 2
Assets:
Assets	215,078	121,891
Recurring \| Level 2 \| U.S. treasury bills and notes
Marketable securities
Fair Value	201,202
Assets:
Investments	201,202
Recurring \| Level 2 \| U.S. treasury bills
Assets:
Cash equivalents		24,904
Recurring \| Level 2 \| U.S. government agency bonds
Marketable securities
Fair Value	13,876	17,701
Assets:
Investments	$ 13,876	17,701
Recurring \| Level 2 \| U.S. treasury notes
Marketable securities
Fair Value		79,286
Assets:
Investments		$ 79,286

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