Fair Value of Financial Assets and Liabilities - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($)	Sep. 30, 2020	Dec. 31, 2019
Assets:
Total assets	$ 399,965,000	$ 0
Liabilities:
Total liabilities	1,940,000	37,690,000
Series B preferred shares forward contracts
Liabilities:
Derivative contracts	1,940,000
Series A preferred shares derivative liability
Liabilities:
Derivative contracts		37,690,000
Money market funds
Assets:
Cash equivalents	167,343,000
U.S. corporate bonds
Assets:
Cash equivalents	2,639,000
Marketable securities	133,959,000
U.S. treasury bills
Assets:
Marketable securities	79,954,000
Foreign corporate bonds
Assets:
Marketable securities	16,070,000
Level 1
Assets:
Total assets	167,343,000
Liabilities:
Total liabilities	0	0
Level 1 | Series B preferred shares forward contracts
Liabilities:
Derivative contracts	0
Level 1 | Series A preferred shares derivative liability
Liabilities:
Derivative contracts		0
Level 1 | Money market funds
Assets:
Cash equivalents	167,343,000
Level 1 | U.S. corporate bonds
Assets:
Cash equivalents	0
Marketable securities	0
Level 1 | U.S. treasury bills
Assets:
Marketable securities	0
Level 1 | Foreign corporate bonds
Assets:
Marketable securities	0
Level 2
Assets:
Total assets	232,622,000
Liabilities:
Total liabilities	0	0
Level 2 | Series B preferred shares forward contracts
Liabilities:
Derivative contracts	0
Level 2 | Series A preferred shares derivative liability
Liabilities:
Derivative contracts		0
Level 2 | Money market funds
Assets:
Cash equivalents	0
Level 2 | U.S. corporate bonds
Assets:
Cash equivalents	2,639,000
Marketable securities	133,959,000
Level 2 | U.S. treasury bills
Assets:
Marketable securities	79,954,000
Level 2 | Foreign corporate bonds
Assets:
Marketable securities	16,070,000
Level 3
Assets:
Total assets	0
Liabilities:
Total liabilities	1,940,000	37,690,000
Level 3 | Series B preferred shares forward contracts
Liabilities:
Derivative contracts	1,940,000
Level 3 | Series A preferred shares derivative liability
Liabilities:
Derivative contracts		$ 37,690,000
Level 3 | Money market funds
Assets:
Cash equivalents	0
Level 3 | U.S. corporate bonds
Assets:
Cash equivalents	0
Marketable securities	0
Level 3 | U.S. treasury bills
Assets:
Marketable securities	0
Level 3 | Foreign corporate bonds
Assets:
Marketable securities	$ 0