Summary of Significant Accounting Policies - Fair Value Measurements - Assets - Tabular Disclosure (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets
Investments	$ 143,459	$ 101,661	$ 98,878
Cash held in bank accounts	464,900	4,700
U.S. Government and agency securities
Assets
Investments	42,399	30,775
Corporate debt securities
Assets
Investments	99,749	69,581
Certificates of deposit
Assets
Investments	1,311	1,305
Fair Value, Recurring
Assets
Cash and cash equivalents	679,554	111,085
Total assets held at fair value	823,013	212,746
Fair Value, Recurring | U.S. Government and agency securities
Assets
Investments	42,399	30,775
Fair Value, Recurring | Corporate debt securities
Assets
Investments	99,749	69,581
Fair Value, Recurring | Certificates of deposit
Assets
Investments	1,311	1,305
Fair Value, Recurring | Level 1
Assets
Total assets held at fair value	42,399	30,775
Fair Value, Recurring | Level 1 | U.S. Government and agency securities
Assets
Investments	42,399	30,775
Fair Value, Recurring | Level 2
Assets
Cash and cash equivalents	679,554	111,085
Total assets held at fair value	780,614	181,971
Fair Value, Recurring | Level 2 | Corporate debt securities
Assets
Investments	99,749	69,581
Fair Value, Recurring | Level 2 | Certificates of deposit
Assets
Investments	$ 1,311	$ 1,305