Cash, Concentration of Credit Risk and Fair Value of Financial Instruments - Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Assets:
Money market and demand accounts	$ 529,725	$ 757,098	$ 516,612
Commercial paper	88,125	0
Total	794,832	936,302
Money market and demand accounts
Assets:
Money market and demand accounts	519,725	757,098
U.S. government securities
Assets:
Securities	86,189	105,702
Corporate bonds, asset backed and other securities
Assets:
Securities	100,793	73,502
Level 1
Assets:
Commercial paper	0	0
Total	519,725	757,098
Level 1 | Money market and demand accounts
Assets:
Money market and demand accounts	519,725	757,098
Level 1 | U.S. government securities
Assets:
Securities	0	0
Level 1 | Corporate bonds, asset backed and other securities
Assets:
Securities	0	0
Level 2
Assets:
Commercial paper	88,125	0
Total	275,107	179,204
Level 2 | Money market and demand accounts
Assets:
Money market and demand accounts	0	0
Level 2 | U.S. government securities
Assets:
Securities	86,189	105,702
Level 2 | Corporate bonds, asset backed and other securities
Assets:
Securities	100,793	73,502
Level 3
Assets:
Commercial paper	0	0
Total	0	0
Level 3 | Money market and demand accounts
Assets:
Money market and demand accounts	0	0
Level 3 | U.S. government securities
Assets:
Securities	0	0
Level 3 | Corporate bonds, asset backed and other securities
Assets:
Securities	$ 0	$ 0